Chapter 1: Rude Awakening: The village discovering Toothless is word-for-word a lot like how Astrid discovered him in How to Train Your Dragon.
Hiccup: He wasn't gonna hurt you, you scared him! Luisa: I scared HIM?
Chapter 10: Cracking Up: The way Mirabel describes her relationship with Alma is the same as how Hiccup described his relationship with his father in How to Train Your Dragon.
Mirabel: Abuela looks at me like with this disappointed scowl, like someone skimped on the meat in her sandwich!
Chapter 12: Room of (Almost) Doom: When their toucan companion flies out of the room in terror, Hiccup yells "Thank you for nothing, you useless bird!"
Chapter 69: Family Reunion: Eret and Arny's revenge plot against the Dragon Riders — planting Whispering Death eggs underneath Berk and let them tear it apart — is taken from Alvin the Treacherous' plan in "Tunnel Vision".
Mirabel and Hiccup trying to leave, only for their guardians to "ground" them for their safety against Dagur on the warpath is a lot like the Dreamworks Dragons episode "Live and Let Fly", where Stoick "grounded" the dragon riders to protect them all from Alvin.
Hiccup and the other riders heading out for ingredients to an Eel Pox cure reflects the episode "The Eel Effect", with the reveal that Dagur infecting their island as a form of biological warfare being similar to when he used Johann to plant a Smokebreath nest on Berk in "Smoke Gets in Your Eyes" and Vigo planning on causing a Scourge of Odin-pandemic for a Poison and Cure Gambit in "Buffalord Soldier".
An In-Universe theatrical recreation of the first installment (in this case, The Dragon and the Butterfly) that goes horribly wrong? Sounds an awful lot like How to Train Your Dragon: Homecoming.
The theatrical portrayal of Toothless being played by a Terrible Terror is an obvious reference to the books, where Toothless is a small dragon not unlike the Terrible Terror.
The things Hiccup and Mirabel rant about their kids are, ironically, things Stoick ranted to Gobber about him; how different Peep was "since she could crawl", how Pedro has the attention span of a Sparrow and Karla's inability to listen to anyone and cause chaos as a result, leaving them to clean up after her.
Hiccup: I swear... It's like every time they outside, some type of disaster follows...
Mirabel also realizes that she sounds just like Alma when she wished her kids would "step aside" so that they could run the town without the chaos they cause.
Chapter 18: The Art of War: The chapter starts with the line "[t]oday's lesson was all about attack", something Gobber says at the start of training day against Stormfly in the first movie.
Drago darkly says "I believe in learning on the job" when he brings Karla and Pedro with him on a mission.
The "challenger" the Northern Alliance finds is a Purple Death, a Titan-class alpha-species that only appeared in the Dragons: Riders of Berk comic book tie-in from Titan Comics.
Alternative Title(s):The Dragon And The Butterfly, Dragons Butterflies And Who Knows What Else, The Dragon And The Butterfly Whiteout