Not to be confused with Pukū
Bukū (ブクー) is a prototype design for Qwilfish which was revealed prior to the release of Pokémon Gold and Silver. Bukū has visible fins not viewable on Qwilfish's final design and is depicted with lighter blue colors than Qwilfish. It has a smaller tail than the final Qwilfish. The underside of Bukū's body is also the same color, unlike Qwilfish.
Bukū supposedly appeared in the Nintendo Spaceworld 1997 demo of Pokémon Gold and Silver. 
Derived from "fugu", the Japanese word for "blowfish".