Soo, RPGs where you can fuse with other monsters and / or take on a were-form?
I'll start with Battle Frontier
( https://linktr.ee/BattleFrontierGame )

Breath of Fire series featured something similar.
and while not a fighting game, "Bloody Roar" .

>>3794397 (OP)
Shadow Hearts

>>3794397 (OP)
Baldur's Gate 2 (Shapeshifter druid kit)
Neverwinter Nights (Shapeshifter Prestiege Class)
Disgaea 3 (not a Merge per se, but you can turn mosters into weapons for Demonoids)
SMT: Digital Devil Saga
SMT: ...if
Summon Knight: Swordcraft Story 2 has shift mode.

>>3794397 (OP)
Jade Cocoon -

Summon Knight: Swordcraft Story 2 had that? Gotta play that game again... I can't even remember that detail.

>>3794397 (OP)
Pretty cool its like pokemon reburst made game
>were form
Diablo 2