While we're loosely on the topic of fan games and hacks, I feel it's worth linking
FusionDex.org for this thread's enjoyment. It's a compilation of Infinite Fusion sprites that has some stats attached and credit for nearly every sprite. Some of this shit is absolutely killer, and the ENORMOUS possible dex (221k+ individual mon) is more than halfway complete with custom sprites by now so you could keep scrolling through it for hours and still find new stuff. Any lack of quality is made up for with sheer quantity, and there's hardly much of the former to begin with.
Would seriously recommend checking out the following mon for some truly cool stuff.
- Any member of the Porygon line
- Jirachi
- Any member of the Clefairy line
- Torterra
- Cofagrigus (and Yamask to a lesser extent)
- Mimikyu
- Any member of the Klink line
- Golurk
- Duskull
- Parasect
- Beldum
- Exeggcute
- Aegislash
- Mawile
Would recommend
against Miltank since its sprites involve a lot of "this Pokémon is lactating and
really enjoying it" shit that is probably fetishistic, with a bonus warning against Sylveon because the people making these sprites come from places where it's more of a trans icon than it is an actual Pokémon.
Otherwise, it's not really worth looking through the sprites of Pokémon that can't be spun into vessels for broader themes or broken up into easily-replaceable parts. Mon like Dugtrio or Blastoise have obvious spots where you could stick new theming (change Blastoise's cannons, make something other than three moles pop out of Dugtrio's hole), whereas mon like Raticate or Dragonite really can't be fused into anything very interesting. Klink is a dogshit mon on its own, but if you use it as a sort of "ingredient" for any other mon then you get mechanical versions of stuff with Klink theming. Porygon is a computer virus that looks like a low-poly duck, but if you fuse it with something else then it becomes able to take on the form of ALMOST ANYTHING. This tends to get used for a lot of cheap references to memes or other games, because Discord, but there's a lot of genuinely funny stuff in there too and a lot of the sprites were very obviously made with love.
If you really like someone's sprites it's worth looking them up on google/ddg/whathaveyou because a lot of the people contributing to the repository have easily-traceable social media accounts or other places to view their work. I've found quite a few neat pixel artists this way.