Master Splinter

Unarguably a very famous rat indeed, Master Splinter's origins can be described, at best, as a
little fuzzy.  Now, if you read the comic version of Teenage Mutant Ninja Turtles (TMNT) you will know
that Splinter was originally the pet rat of a martial art expert called Yoshi.  After killing a love rival, the duo
escaped to New York where Yoshi was killed by his rival's brother (Shredder).  Living in the sewers now alone,
Splinter encountered 4 baby turtles and some radioactive waste. Bish-bash-bosh, and the 5 animals are covered in
slime, growing in size, anthropomorphising
before our very eyes and learning to speak (English or Japanese, depending on where you're
from).

Now, if you're more familiar with the cartoon you will know that Splinter WAS Yoshi who fled
to New York after WITNESSING a murder and went to live in the sewers (as you do). Having
found 4 baby turtles he adopted them and when they ended up covered in ooze the turtles
turned human like because Yoshi was the last living thing they had touched, and Yoshi turned
into a big-ass rat because that was the last living thing HE had touched.

Simple, really.