Notes:

on my deviantart
find the full list of fusions here

so from what i could gather (i pretty much missed this run completely), these were from a twitch plays pokemon playthough called "fused crystal". that romhack existed before tpp played it, but for the tpp playthrough the sprites were made to be actual fusions instead of only recolors. if you want more information, your best bet is probably to visit the subreddit r/twitchplayspokemon and lurk and/or ask around.