I thought I would remember that Seahorses name if I saw it but I looked up a character overview of the movie, and from looking at the character names, it still didn't come to me.
So,here's a list of the names of the characters.
Which one is the Seahorse?
Cast overview, first billed only:
Albert Brooks .... Marlin (voice)
Ellen DeGeneres .... Dory (voice)
Alexander Gould .... Nemo (voice)
Willem Dafoe .... Gill (voice)
Brad Garrett .... Bloat (voice)
Allison Janney .... Peach (voice)
Austin Pendleton .... Gurgle (voice)
Stephen Root .... Bubbles (voice)
Vicki Lewis .... Deb/Flo (voice)
Joe Ranft .... Jacques (voice)
Geoffrey Rush .... Nigel (voice)
Andrew Stanton .... Crush (voice)
Elizabeth Perkins .... Coral (voice)
Nicholas Bird .... Squirt (voice)
Bob Peterson .... Mr. Ray (voice)