The Thirteen Monsters are a collective group of otherworldly creatures that were unleashed via a portal opened by Yaotl three-thousand years prior to the events of TMNT. Yaotl was cursed with immortality, and only by reuniting the thirteen monsters, would he be able to finally perish in peace.
Presumably because of cuts in the film made in order to shorten the running time, very few of the thirteen monsters are seen. Others are only glimpsed briefly.
The monsters that appear with some prominence in the film include:
- Bigfoot: The Turtles first encounter the Bigfoot as Karai and her Foot Ninja are attempting to capture it for Max Winters.
- Centaur: The huge cycloptic muscular quadruped that the Foot ninjas encounter in a warehouse.
- Sloth Monster: The reptilian sloth-like creature that walks on the train.
- Spider Aracknor: The small green arachnoid quadruped captured on top of the Brooklyn Bridge by General Serpiente.
- Vampire Succubor: The tall black bat winged creature that attacked Raphael and Casey on building tops.
- Jersey Devil: Raphael, in the guise of the Nightwatcher, faced the Jersey Devil in the kitchen of a diner.
- Sea monster: A giant creature from the ocean. It eats fish and strongly resembles a dinosaur.
- Winged Portal Monster: The fish-like creature with scaly blue skin and wings with large black eyes and a small snout with sharp teeth and a tail with three tips that was captured by the Foot ninjas. Some of these creatures were also seen in the final battle, but they were sent back to the portal by Splinter.
- Unknown Creature #1: Its legs were only seen.
- Unknown Creature #2: It was seen roaring with its mouth up close and its sharp teeth are shown.
- Unknown Creature #3: Its tentacles were only seen.
- Leonardo: The Stone Generals captured him when they wanted to stay immortal (the reason they do this is because Max Winters wants to break the curse). They think Max will go along and think he is the monster.
In the book The Legend of Yaotl, a complete list of all thirteen monsters is given; however, since it omits the Centaur, Spider Aracknor and Sea Monster, this list is likely not completely canon to the film.
- Lethargo (Sloth Monster)
- Succubor the Vampire Bat
- Jersey Devil
- Vix the Fox
- Lupa the Werewolf (makes a reappearance in Leonardo Returns).
- Pict the Boar
- Azooli the Puma
- Weeb the Frog
- Arkadin the Coatimundi
- Cretaco the Crocodile
- Spitex the Llama
The Bigfoot is a boss in the console/PC and PSP/DS versions of TMNT, as well as the only boss of TMNT: Ninja Adventures. The Jersey Devil and Sea Monster are bosses in the Game Boy Advance version of the 2007 game.
There is also a cutscene of the console/PC version that had a footage from the film. There were some unused scenes from the film, such as the Jersey Devil jumping out of the portal and attacking a soldier, the Centaur and Sloth Monster smashing an opponent with their torsos, and the Vampire Succubor grabbing one soldier and lifting him high into the air.
Standard action figures were released for Sloth Monster, Spider Aracknor/Jersey Devil, and Vampire Succubor. This version of Jersey Devil was included in the Monster Action Tower playset. A smaller Jersey Devil was included with Monster Capture Raph.
Bigfoot was originally supposed to be available in a two-pack with Splinter, but this set was nixed. Instead, Bigfoot became a mail-order premium figure that was available through the end of 2007.
Although not part of the Thirteen Monsters collective due to it being released much earlier, there was a Raphael vs. the Yeti set produced in 2005. This Yeti was a repainted version of the Primal Rage toy of Blizzard.