Acting for Two: Quite common for two characters to share a voice actor. For instance, Bebop and Rocksteady were done by Donatello and Leonardo.
Adaptation Overdosed: In addition to the numerous animated adaptations and video games, there are also the newspaper strip, a series of British-made short comics, two anime OVAs, two live action musical specials, a couple of novels, several kid's books, magazines and crossovers and heaven know how much toys and merchandise.
Adaptation Sequence: Comic —> Cartoon —> Comic based on cartoon —> Movie —> Band —> And so on...
Cash-Cow Franchise: 30 years still going strong and getting lots of merchandise!
Channel Hop: The first three films were distributed by New Line Cinema (the trilogy from the 90s). For TMNT, it was distributed by Warner Bros. and The Weinstein Company (TMNT). 2014 reboot and Out of the Shadows were distributed by Paramount and Nickelodeon.
Executive Meddling: Some cases, such as the 1987 cartoon making the property family friendly (with collaboration from the creators) or adding a female Turtle in Ninja Turtles: The Next Mutation (only Eastman was OK with it, ultimately leading to the rift that led to Laird purchasing his half of Mirage Studios).
Exiled from Continuity: Venus de Milo was barred from ever showing up while Laird owned the franchise, due to him completely hating the character. With Nickelodeon's acquisition of the franchise, it finally became possible for the character to appear again, finally returning in 2022, in the IDW continuity.
Franchise Ownership Acquisition: Co-creators Kevin Eastman and Peter Laird founded Mirage Studios in 1983 order to publish the stories of the heroes in a half-shell and other comics. While the Teenage Mutant Ninja Turtles would be subject to Channel Hop throughout the years, Eastman and Laird would remain at the helm of their comic adventures and hold the rights over the broader franchise. They would sell the franchise in 2009 to Paramount Global while the comics (as one of the terms made in the binding contract to sell TMNT) would continue to be made by IDW Publishing.
God Never Said That: Peter Laird's hatred of Venus de Milo from Ninja Turtles: The Next Mutation is quite well-known, but many fans have mistakenly believed that the franchise's other co-creator Kevin Eastman had the same opinion on Venus. According to this interview, Kevin Eastman actually likes Venus as a character and hopes for her to make a comeback someday. She eventually would make a comeback in the IDW comics.
Pet Fad Starter: Lots of kids bought pet turtles. Few were ninjas, fewer were mutants, and those that weren't kept were often released into the wild. This contributed to how the Red-Eared Slider, a popular turtle in the pet trade, was introduced into many areas it's not native to.
Prop Recycling: Inverted and played straight for the Playmates toylines- sometimes they would reuse molds from other lines (the Muta Force exoskeletons were retooled Exosquad toys), and sometimes TMNT molds would be reused for unrelated lines (the Pizza Thrower was reused for the toyline based off Chicken Run, oddly enough).
Science Imitates Art: Ninjemys is a species of giant, horned stem-turtle from Australia, the scientific name of which translates to "ninja turtle", stated in its description to be "in allusion to that totally rad, fearsome foursome epitomizing shelled success".
Sleeper Hit: Who'd have guessed an independent comic which emerged as a joke between its creators would make both millionaires as it became an international sensation?
In 2015, an alternate universe prequel to Teenage Mutant Ninja Turtles, Foot Clan Origins, was pitched, with Eric Robles of Glitch Techs on-board, which would have been about a young Hamato Yoshi and Oroku Saki, orphans in this continuity, establishing the Foot Clan.
Back when the Archie Comics series was selling well, there were plans to make a Spin-Off of the 1987 cartoon based on the Mighty Mutanimals, a group of allies of the Turtles in the Archie Comics. Krang was planned to be the Big Bad with the Turtles making guest appearances. A whole animation bible that featured designs for characters from the comic series and new characters created specifically for the show was submitted to Ruby-Spears, but the show quickly fell through. The cancellation of the Mutanimals animated series was what led to the team being Killed Off for Real in the Archie series at the hands of the Gang of Four.
Donatello was originally going to be called Botticelli.
Both the second and third movies had removed bits of dialogue explaining where Casey was during the second film. Unfortunately it seems the dialogue never went any further than "He was out of town."
Had the third movie not bombed, the fourth movie would have been about the Turtles and Splinter furthering their mutation. One of the most notable being Mikey taking a more human-like appearance, allowing him to go to the surface. In addition, a fifth turtle named Kirby would have joined the team.
With Nickelodeon owning the franchise now and producing a Continuity Reboot franchise, we likely will never see a true sequel to the 2007 film. All we have now is hints to a plot of a sequel that will probably never materialize.
The 2007 film's director, Kevin Munroe, notes that he would have loved to do a sequel exploring the Foot Clan a little more. He also states that he would also liked to have done something with the Rat King or the Triceratons.
Peter Laird had theorized that had he and his wife passed away before he managed to sell the franchise to Nick, their daughter Emily would've become the owner of the franchise.
The second volume of Tales of the Teenage Mutant Ninja Turtles revealed what type of turtles the eponymous ninjas are: specifically, they are red-eared sliders (or Trachemys scripta elegans). However, some adaptations opt to make each of the four distinct - Rise of the Teenage Mutant Ninja Turtles has only Leo as a red-eared slider.
, Kevin Eastman actually likes Venus as a character and hopes for her to make a comeback someday. She eventually would make a comeback in the IDW comics.