Robbie Rist is a registered trademark?
 

On the TMNT 3 credits, he's listed as "Robbie Rist ®" Umm... I didn't know you could trademark your own name. What purpose does that serve?

I just got a sense of deja vu, I hope I haven't asked this before.

If it's a gimmick name you can trademark it. Think of it like The Rock. His name is Dwayne Johnson, but WWE owns the name The Rock. So if Robby Rist is a stage name, it's possible he reserved the rights and all that good stuff for some reason.