In game terms a creature is:
A model
Basically, yes, but often there are more models (.mdb) files: heads, wings, tails, and also variants both in terms of armor (_CL_, _LE_, _PF_, etc.) and appearance (body01, body02, etc.)
A texture
Actually, you'll need at least two: the diffuse texture (the one you see in game) and the normal map (used for light reflection and bumps highlighting).
Two others are sometimes used in addition (not necessarily at the same time): a glow map (which makes the creature shine) and a tint map (to allow changing colors of the diffuse map).
Some animation files
A sound file
A blueprint
Yes. Note that the blueprint may also need other blueprints (creature items: weapons, like claw, bite, or even Balor whip; armor: hide, adding properties to the creature like damage reduction, vulnerabilities or immunities, etc.)
Without any sound file, the creature will not crash the game, it will just remain silent.
A placeholder in a 2da or more than one 2da?
At least in appearance.2da.
But if there are tails or wings involved, tailmodel.2da and wingmodel.2da will also need a line to define them.
racialtypes.2da and racialsubtypes.2da might also be involved for new playable creatures (if they're obviously from a different race than the usual human, elf, dwarf, gnome, etc. list).