In TMNT #8 Baxter hints that he needs to get hold of Splinter, no other rats would do as he was the only proper lab experiment. It makes me think how're the mutants differ from each other (Splinter, the Turtles, the other mutants...)?
There was only one way that mutants were created, all of the other strange creatures were born that way. Splinter was actually the only "mutant" that was different, by virtue of his extra-ordinary intelligence. Splinter wasn't as mutated as the others, however. He was a genetically extra-ordinary rat before the ooze, when he learned speech and when he learned martial arts, and very little of the mutagen he encountered affected him: He only got it on his hands when cleaning the Turtles. His DNA being far less affected by the ooze is why he was the best test subject.
Note that the asker was asking about the comic and not the movie. In the comic, Splinter was always a rat.