What is a pregnant goldfish called?

A twerp.

It's number 131 in the list:
http://dogman0.tripod.com/useless.html

Of course, the whole idea of a pregnant goldfish being called a twit, twat or twerp is a joke based on there being no such thing. It has been gullibly repeated by countless ill-researched trivia lists, but I'm pretty sure that neither twit, twat nor twerp have anything to do with fish in most reputable dictionaries or books about fish.
Just use "Onelook" to check out these words in a number of dictionaries if you don't believe me. Here is a site with an interesting theory about where the word "twerp" in its usual sense of an annoying git might have derived:

http://www.word-detective.com/120505.html

Goldfish lay eggs! A guy at the Goldfish Society of Great Britain had never heard of one being called a twit.

twit [twit] noun: A "pregnant goldfish"; which is impossible. Some species of fish do give birth to live young (e.g. guppies and some species of shark), but goldfish, like all cyprinids lay eggs.