Adaptation Decay: The inevitable result (to varying degrees) when the stories are brought to stage, radio, film, and comics. Some particularly picky ponies refuse to read or listen to any of the spinoffs.
Alas, Poor Villain: When Inferno believed Ahuizotl smashed her egg, she went on a rampage forcing the fleet called in as reinforcements to shoot her down. When Daring showed that the egg was fine, she said she was sorry, and asked Daring to keep it safe before collapsing.
In Mirror Pond, being forced to admit the truth Greyhoof has been evading for so long to create the Cutie Pox cure (and realizing there was one) broke him on a fundamental level. He drank himself to death right afterwards, using a combination of some 470-proof hard apple cider and a torch.
Alien Geometries: Steam Whistle's workshop. The straight hallway that loops is probably the tamest anomaly.
All Myths Are True: The series makes heavy use of both Equestrian and non-Equestrian mythology, including the more mythological aspects of real historical artifacts. However, with the release of Nightmare Moon and Discord (who matches the description of the Great Beast Of Chaos in Alicorn's Shadow), there is talk that maybe the myths really are true.
Subverted with "The Poodonkus". That one was revealed to be false.
Something of a weird case in Tinker's Seal, where Star Shimmer is vaguely implied to have been a descendent of the Dark Lord of Ultima Mule. However, after the return of the Crystal Empire, scholars have noted that Ultima Mule had many trappings of the Crystal Empire (crystal motif, evil ruler, lost in time). That said, Ultima Mule was supposedly banished because it was just as wicked as its ruler, whereas the only true evil in the Crystal Empire was King Sombra and his personal retinue, and it was cursed to be lost in time by King Sombra as a final "buck you" to the Princesses. Word Of God decided to go with Star Shimmer being a descendant of King Sombra and one of his dark consorts when asked about it.
Alternate Universe: Multiple are seen in the Expanded Universe story Universal Cracks. Most notably, the one where Daring died in place of Zapapple Tock. Others seen include one where Daring and her friends wield the Elements of Harmony, a Genderswapped Universe where Daring is a male, and the Bizarro Universe that serves as the origin of the Leader of the Colt of Smooze.
Always Somepony Better: Derring-Do has repeatedly proven that she's a better explorer than Daring is. Interestingly, Ahuizotl can't stand her and when she gets involved in one of his schemes, instead of Daring, he forgoes the usual Death Trap and, instead, curbstomps her and takes the artifact. Daring must then retrieve it along with rescuing her sister. Ahuizotl claims he could do the same to Daring as well, but chooses not to, since it's more fun to struggle with her.
Subverted in Shrine of the Silver Monkey, when both Derring-Do and Daring Do get placed in identical death traps. Daring escapes easily due to her prior experience with such things while Derring needs to be rescued.
Alchemy Is Magic: This trope is played completely straight with Elementum in Daring Do and the Sultan's Curse.
Anachronic Order: The novels, as mentioned above, though some stage and radio adaptations have averted it, choosing to begin with Spear of the Windigos. The Broncway plays stuck with the books' order while Hairerion Trot was in the role, as did the first seven Applewood films before various circumstances resulted in the producers fiddling with the schedule: Shrine of the Silver Monkey was moved forward due to Pony Jay's failing health, Staff of Star Swirl the Bearded was moved up due to the success of Sweeney Trot's Broncway run, Trident of the Seaponies was delayed as the amount of swimming and diving training involved would have put the film months behind schedule, and, most controversially, Alicorn's Shadow was cancelled during production due to a falling-out between Agister Jolie (Derring) and Brad Pinto (Mareton). This resulted in Mareton being written out of the Temple of Nightmare Moon. Both of these are back on track, however, thanks to Hairerion Trot taking lessons on her downtime, and Haystack Leaves being cast as Mareton.
Androcles Tyrannosaur: Near the end of The Valley of Grouchy,the eponymous dinosaur swallows Daring whole before he is subdued by the royal guards. Daring discovers a bone stuck in his throat and dislodges it, for which Grouchy is extremely grateful.
Apocalyptic Log: Subverted by Steam Whistle's journal. Both Daring and Herpy expected it to end on a catastrophe, but the last entry is simply Steam Whistle expressing his hopes that "future generations understand that we did this for them" and casually mentioning he plans to get himself scrapped once he's done writing it.
Arc Words: Throughout Trident of the Seaponies, the phrase "Sea bears foam, Life bears dreams; both end the same way: CRASSSH" are uttered by both the old oracle an during the Dream Sequence. This on its own is a subtle shout out (or probable origin, considering the uncertain time of writing of the series as a whole) of a phrase used in an unrelated work: The Awakening of the Wind Fish
A less obvious example is in Cove of Candles. Every clue given to Daring regarding Blackmane's treasure contains the phrase "You have to go inside", from the obvious, to the spelled out through misplaced capitalization to written in invisible ink on the scroll.
Two of the villains in the Expanded Universe have Arc Words associated with them-books involving The Smooze often contain the phrase "Nothing can stop The Smooze", while books where Krastos appears have "All is one, one is all", referencing his final Grand Theft Me plan.
The phrase "Someday I'm gonna go home" is an Arc Phrase that appears throughout the Colt of Smooze/Colt Leader subplot in the Expanded Universe. This is brought to a conclusion in The Stinger for Tinker's Seal, which leads into the Colt Leader-centric Universal Cracks: "I'm going home."
In Tales of the War, the phrase "End the War Tonight" appears frequently in the story Sacrifice of Alicorns, and to a lesser extent, No Matter The Cost.
"Oh, come on, Sweeney! Even if the artifact's power could bring back your wife, would she even want you after all this? Would Linnet?"
Artistic License- Equestrian History: The series ping pongs on it. For the most part the actual Royal Canterlot Equinology Society praises the series for its accuracy (right down to the number of bells on the hat of acclaimed wizard Star Swirl The Bearded), but they do note that some of the twists are impossible and directly contradict facts, most notably in the Nightmare Moon arc, where the villains' plans revolve around channeling power directly from the eponymous goddess in her prison. Ahuizotl, in order to destroy Canterlot with the use of the artifacts Daring has been collecting; and Inti, simply destroying much of Equestria and reshaping it to his liking. It's noted that channeling such power would be impossible due to the nature of the seal on Nightmare Moon and the fact that many of the cultures the artifacts come from predate Nightmare Moon's imprisonment.
Also from Temple of Nightmare Moon, the eponymous Temple is said to have been Nightmare Moon's main fortress when she began her campaign against the Sun. While there was a Real Life "Temple of Nightmare Moon", in reality it was just Princess Luna's (rather elaborate) summer retreat home converted into a place of worship for Nightmare's followers and served no military value during the Equestrian Civil War.
One of manuscripts contained a series of frustrated notes, where the author debated telling a good story versus accuracy. Apparently, the author went with a good story.
Ascended Extra: In The Griffon's Goblet Daring briefly stops to talk to Herpy, a somewhat silly, but surprisingly competent Page for the University. Later on he becomes more prominent, becoming Daring's assistant and helping out with her research and the like. He is Put on a Bus in Cove of Candles where Daring remarks that he is working on getting his own degree in Equestrian Economy. He comes back for a minor Big Damn Heroes moment in Temple Of Nightmare Moon and later gets A Day in the Limelight for a large chunk of Ring of the Marengeti. In "The Spear Of the Windigos", although not mentioned by name, Daring complains about how a pony matching Herpy's description drank all the coffee.
Mahavir/Mahiavar was only given a passing mention in the main series of books is given a prominent role in the Expanded Universe
Swinn and Dell, a pair of traveling Con Artist sisters who had popped up as joke characters for several volumes, actually help Daring in Cove of Candles, and again with several other characters in Temple of Nightmare Moon.
Dr. Capacitor's cousin, Lemon Johnson, originally given a brief mention in Tinker's Seal, becomes a Supporting Character in Universal Cracks.
Ass in Ambassador: Ambassador Hawkwings is a rude, obnoxious and outright abusive guy who only really got his job via nepotism, and who by the penultimate chapter of Griffon's Goblet only avoids a righteous beating by the Dos by claiming it could create a serious international incident. At which point Professor Storm Talon casually walks by and smacks him over the head with his prosthetic tail.
Author Appeal: Expanded Universe author L. Heartstrings has a weird obsession with hands, the appendages where the forehooves would be on Minotaurs and apes. She's also very big on the mythology of humans (think Incitatus' unique ape, Calligula, or the series "My Small Human").
A few excerpts from Alicorn Amulet indicates that Beatrix Lulamoon has an interest in vanquishing Ursas and the Magical Filly genre.
Every few chapters in Mystery of Flutter Valley, a stallion is rolling. And Herpy rolls in the Crystal Message preview.
Author Avatar: Fans have searched the books to find a character who might be operating as one of these. The most intriguing (because the most Badass?) is Laurentia of the Red Mane from ''Temple of Nightmare Moon".
Another possibility is Mirror Dreams, who first appeared in Alicorn's Shadow. Mostly due to her freaky knowledge of everypony's backstory.
A third option is Herpy, which is brought up because of another rumor that the author's major is in Equestrian Equinology and that s/he based Daring Do off of a pony s/he was a grad student to.
Author Existence Failure: Gusty Lulamoon was slated to write a sequel to Ruby of the Blank Village, before she was found dead in suspicious circumstances in the Everfree Forest. Her notes were left to J. Thunderlane Hurricane Jr, who used some of her ideas in Tinker's Seal and Mirror Pond.
Haystack Leaves has died, before the release of the film of Alicorn's Shadow.
Awesomeness by Analysis: Herpy's modus operandi when it comes to death traps. While Daring Do simply charges ahead and dodges by reflex, he tends to study trajectories or find ways to trigger or disarm the traps.
Batmare Gambit: After losing the first few artifacts necessary for his doomsday weapon to Daring Do, Ahuizotl starts letting Daring Do retrieve the other artifacts, so that she'll gather them all in one place. He can then swoop in and steal them all from her at the same time.
Badass Damsel: Holy clopping horseapples, Shūbidū in Vault of Posteidon.
Badass Family: The Dos. Unquestionably. Griffon's Goblet and Trials of Unity both showed how unstoppable this family can be when they come together.
The Bad Guy Wins: Silver in Cloudfall Conspiracy - Masra is killed, and her reputation is so badly tainted that the University board decides to go in an entirely different direction with her replacement. That said, he doesn't live to enjoy said victory.
Batmare Gambit: Ahuizotl's plan to get rid of the Do Siblings in The Trials Of Unity. He found an ancient treasure that destroys (and teleports) anything around it whenever it comes into contact with sunlight, and moved it to an equally ancient temple that hosts the eponymous Trials Of Unity: a series of challenges that are magically enchanted to only be solvable by siblings. He placed the treasure within the trials and sent a map to Daring's sister Derring. He knew that the sisters' obsession with finding treasure would make them willing to co-operate, but he also planned for their rivalry to interfere with their performance in the trials, leading to their destruction.
Xanatos-Gambit:Even if they somehow succeeded, the treasure waiting for them at the end would also destroy them as soon as they took it outside. Whether they win or lose the trials, Ahuizotl wins.
Take a Third Option:Fortunately, the Do siblings deduce the nature of the treasure just in time and throw it into Ahuizotl's arms as it activates. Being Nigh Invulnerable, the treasure doesn't destroy him, but it still teleports him far away where he can't immediatley threaten the Do Sisters.
Various other villains take this role in the Expanded Universe books, among them being the Colt of the Smooze and Krastos.
Tinker's Seal has a veritable Big Bad Ensemble, with Ahuizotl, Krastos, and the Assembler all fighting for control of Steam Whistle's creations.
Big Damn Heroes: Herpy gets an awesome one in the Ring of the Marengeti when he saves Daring Do. Who knew he had it in him?
This is how Derring-Do is introduced in The Platinum Crown. In the beginning Derring-Do saves her sister from an especially nasty death trap and carries her to safety... only to incessantly rub it in Daring's face after the fact. Later on, Daring gets a BDH moment of her own when she saves Derring from Ahuizotl and his forces.
The climax of The Valley Of Grouchy involves a reconciled Daring and Grouchy saving Derring from Hammond's regiment and throwing Hammond into prison.
Sweetie Bottle in ''Universal Cracks.]]
Big Little Brother: Daring and Derring's younger brother Darrin. He appears to handle the logistics of the operation, while Daring and Derring do the actual adventuring.
Big "WHAT?!": Once per volume, and always by one of the main cast.
Played straight in the original canon series, not so much in modern interpretations.
Bizarro Universe: The existence of one is hypothesized by Daring as the origin of the Colt Leader. Proved in Universal Cracks to be correct.
Black-and-White-Coat: Rare non-racist example in Alicorn Amulet; In order to simulate turning a stallion into a mare, Herpy has Daring painted up to look like a female version of Okpono.
Bloodier and Gorier: Blood Diamonds is this, though not so much that the books have gratuitous carnage, but the source material is clean enough that even moderately violent adaptations are this trope.
Blooper: One of the print runs accidentally put the title for Quest for the Sapphire Stone on the cover of Griffon's Goblet while keeping the correct picture. While it was caught in time, the books were distributed as collector's items at a convention.
Animal Assassin of every kind imaginable: scorpions, snakes, spiders, you name it. It crawled out of walls of a trapped room.
Book Ends: The Blood Diamonds arc begins and ends with Daring giving Storm Talon a Death Glare. In the beginning, because of a Noodle Incident, and at the end when Storm Talon thinks she's been on vacation.
Bound and Gagged: It's not uncommon for Daring and her friends to be left tied up on top of whatever death trap they're stuck with, just to make it that much harder.
Broad Strokes: The Tinker's Series made a concerted effort to tie together as much of the Expanded Universe as possible (except Buffalo Burial Grounds), but since Expanded Universe works were not mandated to fit together except within their own sub-series, there are a few things that had to be shuffled around. For instance, Ruby of the Blank Village and Scepter of the Chaos Beast take place over the same period in vastly different areas, so the assumption is that one happened after the other in the production order. Meanwhile, Silvia Clawson is stated to be Hawkwings' illegitimate daughter in Eye of Discord, but she's his mistress in Burning Heart. The Tinker's Series hangs a lampshade about these two positions but doesn't actually take a definitive stance.
Brown Note: The Princess in Pink. Little is known about the play, other then it involves a costume party, the Vivid Balloon, the Pink Mark, and is freaking hilarious. Herpy reads it and ends up laughing uncontrollably, all throughout a Daring Escape no less.
Minuette:(Delighted, aside to Lyra.) No mask? No mask!
It is believed that this cursed play, when performed in its entirety, is a summoning ritual for Hosstur, He-Who-Must-Not-Be-Neighed.
From the Expanded Universe: The Song of the Smooze. It gets in your head and tells you "Nothing can stop the Smooze", and those who hear it are compelled to forward its agenda.
In Amber of the Smooze, after taking over Desert Rose's body, one of the first things the Colt Leader does is screw with Gypsy Bard, who had just made friends with Desert Rose and could distinguish the very subtle visual differences between them.