I saw Hellboy last week and was sure the voice of the fish-guy was David Hyde Pierce.
Credits rolled and no mention of his name 
Checked the IMDB and it was his voice but he’s uncredited.
He voices one of the four main characters and is still uncredited.
Weird.
Sometimes an actor will be unbilled because of pending contracts with studios. If an actor has a three picture pay or play deal for say 20 mil a picture, the studio has agreed to make three pictures with this person. However if a friend of the actor has a project that they want to do a favor for (and they are willing to work for peanuts), they will do the film uncredited because if they appeared in the credits as an “official” cast member, the studio would have to pay them 20 mil for a bit part. (Because of the contract) Or an actor will believe in a project, or a franchise so much that they just want to be a part of it. I understand that is what Whoopi did with Star Trek. SHe just wanted to be a part of the expierience.
At the time the movie was made, due to several succesful movies, a TV show of her own, several books, and her recurring role as Guinan, Whoopi Goldberg had the highest recognition value, or whatever they call it, for actresses. In other words, show a random sampling of people a bunch of pictures, one of them hers, and more people would recognize her than any of the others.
This was also around the same time that Patrick Stewart was voted one of the sexiest men alive…
STNG was big, people. That cast has gone on to do some amazing stuff.
I believe that Emilio Estevez was uncredited for his bit part in the beginning of Mission: Impossible.
I’m pretty sure David Hyde Pierce played (or at least did the voice of) Abe Sapien in Hellboy, but he wasn’t in the credits.