Clearly that cameo was very memorable. lol
Thinking on it more now and looking online... I can't find a pic of it, but I think it's from the Chris Brandford cartoon where they're climbing those stairs and then the kid sidekick Lil' Rineo surprises him by being some villain in disguise. The villain, according to the wiki page, being the "yellow monster." Presumably the one that looked like Carter.
edit: Ah! Just found a pic.
From
http://de.tmnt.wikia.com/wiki/Requiem (If you read German.)
I REALLY don't want his mutation back.
Also... Great, maybe it's just a coincidence, but 'Rise' Leo pretty much took those red marks from Carter.