#Forgive me for folklore inaccuracies

[tmntkiseki]

My hilariously dumb headcanon on how 03 Splinter was able to learn martial arts from Yoshi despite being a perfectly ordinary rat: He is actually a kind of yokai that has achieved some spiritual enlightenment due to having lived far longer than the average rat.

For some preface: These days, the term tsukumogami applies to any object that has existed for 100 years and has gained sentience, becoming a yokai. However, at one point it applied to both inanimate objects and animals that had lived for at least 100 years and thus changed forms, with the kitsune and bake-danuki perhaps being the most famous examples of long lived animal yokai.

In terms of Splinter: We don't know how long Splinter lived with Yoshi prior to his death at the hands of The Shredder, but most species of rat only live around 2 - 3 years at best in captivity. I can't imagine that 2 years would have been enough for Splinter to have learned and mastered all the ninjutsu from Yoshi that he would later pass down to the turtles. In particular, when Splinter is explaining his origins in Part 1 of the Shredder Strikes, he specifically says "for many years, I was happy." How much is many? I don't know, but my gut says it can't be any less than five years at least.

There is also the matter of Splinter's behavior as a rat. Besides the fact that he was able to learn ninjutsu by simply mimicking Yoshi's moves, there is the amount of human-like empathy he showed even before he mutated. After witnessing the incident involving the TCRI van, he went down into the sewers, took one look at the ooze-covered baby turtles and decided "Well, I'm not just going to leave these poor things here," gathered them up in an empty coffee can, and took them back to his burrow.

And then there's the episode during Season 5 where the turtles learn that they had already been to Japan to bury Yoshi's ashes with Splinter. During the flashback, the Ancient One refers to Splinter as a Nezumi and declares that he has "no time for today for a rat monster's mischief;" in other words, the Ancient One automatically assumed that Splinter was some kind of supernatural being there to play some kind of trick of him (a lot of yokai are known for being tricksters, after all), and we do see monstrous rat demons serving the Tengu Shredder.

tl;dr: Splinter is simultaneously a mutant as well as a yokai.