Pendulum War: Battles between Cats and Lizards are usually shown with the odds heavily weighted in favor of one side, only for the tide to turn dramatically in favor of the other.
Perfect Pacifist People: Deconstructed with the Ro-Bear Berbils. A race of Cute Machines programmed as builders and helpers, their villagers are regularly abducted and enslaved by The Conquedor due to their inability to fight back. Panthro must train them to build a village defense system.
Pig Man: Encountered in the Swordsmans' Town, one a miserable failure at a sword contest, the other a crony of the Duelist.
Please Spare Him, My Liege!: Reconstructed when Lion-O doesn't so much plead for as demand the release of two stockaded Lizard prisoners whose lynching was narrowly avoided by Claudus' timely arrival. Though skeptical, Claudus takes Lion-O's Sudden Principled Stand as an illustration of Lion-O's leadership, and pardons the Lizards.(Besides, he still has plenty in the dungeons.)
Plot Tailored to the Party: Features in "Journey to the Tower of Omens". To whit, Snarf discovers the entrance to the dungeon by trying to eat some fruit from a tree. Getting past the first trap requires the Thunderkittens' lockpicking skills, the second, Cheetara's super-speed and extendable staff. Panthro's Super Drowning Skills in a water trap leads him to find a way out and save the others, Lion-O is able to use Sight Beyond Sight to find a switch in a cliffside, which Tygra activates by hitting with his whip, thus opening the way to the Tower.
The Lizards loot Thundera, and the Thunderkittens, in turn, help themselves to their takings to acquire new clothes.
Captain Tygus procures the Warstone for his Commander as spoils of war.
Pointy Ears: The Cats, many of whom have ear placement and shape that is more Elfin than purely animal.
Politically Incorrect Villain: The Cats of Thundera. Alley Cats like beating and mugging hapless Dogs, and festival goers like yelling at beggar children and tormenting and lynching stockaded Lizards.
Portal Cut: Panthro loses both arms when the portal to the Astral Plane collapses while he's holding Grune on the other side.
Powderkeg Crowd: At a festival, one of these merrily harasses and physically threatens a pair of stockaded Lizards, until Lion-O attempts to shame them out of doing so. Things go rapidly downhill from there.
The Promised Land: The Garden is the ancestral home of the Petalars, which they've been separated from for generations, ever since a great wind blew them into the Briar Woods. They travel in search of The Cliff of Winds, a location in the woods that will mark the way back to the Garden.
Prophecy Twist: Ahmet warns Lion-O that he will feel betrayed by his brother as the evening bell tolls. In the Astral Plane, Tygra almost but not quite betrays Lion-O, seemingly proving the vision wrong. Then, just as the evening bell tolls... Lion-O catches Tygra kissing Cheetara.
Protective Pressure: A tremendous amount is released when Grune forces open a door to Mumm-Ra's suspended animation chamber.
Particular emphasis goes to the Tiger Clan, who refused to ask anyone for help curing a plague. As a result, everyone died, and only a deal with demons made it possible for them to "Live"... but at the cost of turning into monsters every night, and technically being locked in a state between life and death.
Punctuated Pounding: "These bears *clang* are not *clang* your property!" (Cuts Conqedor's weapon in half)
Claudus continues his lecture while sparring with Lion-O.
Puppy Love: Invoked in-universe with Tygra's quasi-courtship of Cheetara when they first meet as children.
Put Down Your Gun and Step Away: In "Into the Astral Plane", Grune orders the ThunderCats to drop their weapons, or his troops will open fire on their Elephant hostages.
Pyrrhic Victory: Leo's rebellion against his Commander freed the enslaved Animals, but at the cost of the loss of their homes and great loss of life as their ship crashes into Third Earth.
Referenced as exact words in "What Lies Above: Part 2" by Mumm-Ra: Lion-O, the other cats, and all his allies have defeated him, Panthro manages to land the falling city mostly safely, and Lion-O has seen how many he has helped and how this inspires hope that all the races of Third Earth can finally unite to stop Mumm-Ra...but he escapes with the Tech Stone, and Pumyra is revealed as a traitor and goes off with him.
Rain of Arrows: The Cats employ this tactic during The siege of Thundera.
Rape, Pillage, and Burn: Mercifully confined to the latter two, this being a children's show, the Lizards are shown sacking Thundera's smouldering ruins.
Rapid Aging/We Are as Mayflies: Inverted: The Petalars have incredibly brief lifespans compared to the Cats, only living for about a day before they wither away, but Cheetara points out that this is relative. To a Petalar, that day is an incredibly rich, full lifetime.
Rat Men: Among the many animal races are sapient rats and mice.
Right Makes Might: This is essentially how the stones of power work. The more righteous the wielder, the more power the stones will grant them. Lion-O and Leo are able to use their full power because they fight for the good of all the races. Claudus is able to use a lot of the sword's power because he fights to protect his people despite being a Noble Bigot, but not to its full extent. The sword stays in its dagger form in Grune and Slithe's hands, and it outright electrocutes Mumm-Ra, who is evil to the core and believes Might Makes Right. Possibly subverted though, and Mumm-Ra believes he can force the stones' full power with his own immense magical strength.
In "Omens Part 2" this prompts the ThunderCats' escape from The Siege of Thundera, as Mumm-Ra, Grune and the Lizards' Lost Technology-fueled military might is too much for the city to withstand, much less a few protagonists.
In "Song of the Petalars" Lion-O first wisely elects to retreat from the pursuing Lizard army, only to later confront them head on in a grief-fueled fit of Honor Before Reason.
Screw This, I'm Outta Here!: The Lizard army has so little loyalty to Mumm-Ra — not that he deserves any since he doesn't care one bit for them — that squads are all too willing to accept Lion-O's offer of co-existence and abandon Mumm-Ra to go back to their homes.
Later, in "The Curse of Ratilla," Ratar-O's servant Mordrax gets rather fed up with his master's frequent abuse, and turns on him at the episode's climax by giving an unarmed Lion-O his weapons.
Screw You, Elves!: In "What Lies Above, Part 1," Tygra doesn't take kindly to bird prefect Vultaire's haughty air of superiority. Rather than argue with Vultaire, Tygra tells Vultaire to shut up and tries to take him down a peg in a air race.
In the first two episodes, the Lizards get a pretty good one in on the cats, who kept them as slaves, by utterly obliterating Thundera with mecha-tanks, missiles, and projectile weapons... while the cats, who were so convinced of their superiority, only have small amounts of magic, physical strength, and medieval-era weapons. As one lizard put it, they (the cats) went from the dominant race to an endangered species in a single day.
Shades of Conflict: White and Grey vs. Grey and Black. Well-intentioned ThunderCats Lion-O, Cheetara and Jaga work with grey-shaded pickpockets the Thunderkittens, hardened cynic Panthro, and Noble Bigots Claudus and Tygra; and all come from a classist, racist culture. Evil Overlord Mumm-Ra has Selfish EvilDragon Grune, while the Lizard army is reacting to generations of the Cats' oppression, and its General Slithe has internalized the Cycle of Revenge.
Ship Tease: Lion-O has had several with Cheetara, but now there's the matter of Tygra's attraction...
Ship Sinking: Though it keeps to the mythos with Cheetara ending up with Tygra.
As of her second episode Pumyra starts flirting with Lion-O, who is just plain confused. And the episodes following it throw in a lovely dose of Belligerent Sexual Tension, culminating in a Smooch of Victory in "Recipe for Disaster."
In "The Soul Sever", Tygra's robotic body looks suspiciously like a certain Autobot leader. And its final form is basically a mechanical version of Jeff Goldblum's The Fly in his final form.
In the same episode, Soul Sever himself is trying to revive his dead family's souls in mechanical bodies like his. This is all fine and dandy until you realize he's voiced by Jeffrey Combs of Re-Animator fame.
Another reference has Soul Sever briefly turn on a vision mode that scans body heat with a blue background, as well including faint sound resembling a heartbeat when it is on,very similar to The Predator's mask vision.
In "What Lies Above", the scene where the Cats first enter Avista City is higly reminiscant of the Millenium Falcon's arrival in Cloud City. Including the music during their first walk through this City in the Clouds.
"Native Son" is centered around people who refused to side with the good guys during an ancient war, were exiled into a remote place, and died out with a dark curse hanging over them. Well, let's hope they come back, swarming over the enemy as ghosts
Cheetara says "Gotta go fast!" before dashing through a hall of blades.
Shrine to the Fallen: During Grune's homecoming festival, Panthro's painted portrait and a stand holding his nunchucks receive a place of honor at the royal family's banquet table in their amphitheatre.
Wilykit and Wilykat's father gets a portrait drawn by his children hung up in the family home after his death.
Sibling Rivalry: Lion-O suffered from cultural ostracism and is jealous of the success and favoritism adoptive older brother Tygra enjoyed both with their people and their father. Tygra sees Lion-O as a Cloudcuckoolander and Just a Kid and resents Lion-O's right of succession by bloodline. Then there's the fact that they both like the same woman...
Sibling Team: There are two sets in the series: brothers Lion-O and Tygra and the twins Wilykit and Wilykat.
Sibling Triangle: About the only interest Lion-O and Tygra share is their attraction to Cheetara. Foreshadowing implies that Tygra may betray him because of it.
Subverted. Tygra doesn't necessarily betray Lion-O but he DOES end up with Cheetara, which, for Lion-O, is probably just as bad.
Space Romans: The Cats wear no togas, but they are a Proud Warrior Race that has lavish Blood Sport spectacles and extravagant feasts in a huge amphitheatre and take conquered races as slaves. And of course, they do love themselves some Lions... Everyone's armor even has a six-pack like the articulated armor of Roman Legionnaires.
Stock Footage: During The Siege of Thundera, the same "arrows fly, Lizards shot at, Lizards run" sequence was used at least twice, quite close together, which is particularly jarring. The Internal Homage to "ThunderCats HO!" has already been reused, even in an area with no cliff.
Mostly averted with Mumm-Ra's Transformation Sequence, however: as of May 12, 2012, Mumm-Ra has transformed four times on-screen, and three different sequences have been used.