Just cause I'm bored I've compiled a list of Castlevania games and monsters since Rondo that share graphics. Games past Rondo began reusing monsters and the percentage THEORETICALLY seems to have taken a considerable jump with the newer games. However, is that really the case? Let's investigate.
Rondo, number of enemies ?, 98% original (the Toad was at least ported and edited from an earlier Castlevania game, I'm almost positive there are others).
SotN, number of enemies, ~146, 70% original (old Rondo sprites mixed well with new sprites sharing same style). 42 reused sprites.
DoS, number of enemies, ~116, 70% original (old Rondo/SotN sprites ... questionably mixed with some pretty eye-sore new sprites). 35 reused sprites.
PoR, number of enemies, ~155, was difficult to keep track honestly, around 35% are original, and I actually kinda stopped counting at or around entry 140 with at least 100 reused sprites.
So, DoS isn't so bad. Is like SotN in that for all the reused stuff, there's plenty of new. PoR is a steep drop in original monsters, at least. On the bright side, at least they're not reusing background graphics. It could always be worse.