When putting a number of two months' old trout fry (S. fario) into a reservoir near here, I noticed one with two perfectly formed heads joined by a web of skin just behind the gills, the bodies merging into one at the dorsal fin.
The after part of the body is perfectly formed, and the fish did not seem to have any difference of opinion as to the direction it was to take in swimming and appeared lively and well. It was brought away and put into a small tank, where it has since died and is now preserved in spirits.
I am informed by the gamekeeper who hatched it that fry with two heads or two tails are not uncommon, but he has never seen a yearling trout with two heads. I enclose a sketch showing side and upper views of the fish, from which it will be seen that one head is about half a head longer than the other.
-- Thomas Winder, Sheffield.
Hardwicke's Science-Gossip, 1886.
It struck me that we often see photos in the Fortean Times of conjoined mammals and snakes. But 'siamese' fish photos are definitely lacking. Perhaps they aren't noticed because they're aquatic, and maybe they don't survive long and just get eaten, unseen by terrestrial apes.