union cave but it is a static Pokemon (meaning it is encountered when interacted with)
he/she is more specifically found in bottom floor 2 of union cave.
as far as my memory beholds, union cave is near slowpoke well to the right. (someone correct me if I am wrong)
hope I helped and you can find this info on the pokebase website.
fyi this is both on gold and silver gen 2 and 4.