Np, i dont think there is unused model, or at least on the PSX
You have to solutions for this, the first and easiest, is to give to vincent limit 1 , another character than gallian beast (werewolf here), like chaos for example, this will end with :
vincents
limit1:chaos
limit2:deathgigas
limit3:hellmask
limit4:chaos
If you want vincent's limit unchanged there is another way
Vincents have a total of 7 models
the 4 monsters , and 3 others : vincent1 with gun, vincent2 with shotgun, vincent3 with rifles
So instead of erasing the gallian beast of vincent, you can use for red13 one of 3 vincents model ( swaped with werewolf)
FOR PSX ( and maybe for PC)
00= gallian
01=gigas
02=mask
03=chaos
04=vincent with gun
05=vincent with shotgun
06=vincent with rifle
Lets say you will use vincents shotgun 3D model (05) for werewolf
The result will be that all vincent's limits will be untouched BUT when u will equip a shotgun for vincent, he will be redwerewolf, to fix this just go in wallmarket and change all the shotguns ID to any gun or rifle ID.
How to do it? notice that in every animation script i gave you there is a value that never change , this is AC 00, just change AC 00 to AC 05 and red13 will transform into vincent(shotgun) , now just swap vincent shotgun model with redwerewolf, and ofc restore the default gallian beast model.