So...what exactly did Carter mutate into?

[Jester]

So, the mutagen that Shredder used to "create" the Turtles and Splinter caused "who ever touched it to take on the form of whatever animal they had most recently been in contact with." Now I know the mutagen was unstable, resulting in the Turtles' secondary "super" mutations. But what exactly was Carter when he "mutated"?
(Also: Donnie certainly had a lot of Shredder's left over mutagen. Couldn't they have used it to create an anti-mutagen for Splinter? And why didn't he have a secondary mutation?)