Someone beat me to Charizard. It looks like what you'd get if you gave Dragonite a Fire typing. Speaking of Dragonite:
This is Charmeleon's back sprite form the Generation I games. Looks exactly like a Dragonite, in my opinion.
Yeah, it looks almost like a direct copy with some minor edits, but while you could blame it on hardware limitations, it dosen't even have a horn. It's a protrusion of sorts, and shares the same color as it's skin.
Kadabra's sprite in R/B looks like an Alakazam with a missing spoon and a slightly larger tail. No defining features, like the star on its forehead.
It feels mean of me to kick down the first generation while it's down, but...these are just bad.