Procedural mobs would be fun. I still think horror mobs should be a mod, not in the main game.
However, here are some ideas for mob "parts". (with maybe an upper limit on the number of parts per mob creature? at 17 or so?)
Running Leg (adds 2m/s to the running/swimming/floating speed) +2 hp
Jumping Leg (adds 3m/s to the jumping power and reduces the jumping recharge time by 10s from a maximum of 3 minutes or so.) +2 hp
Claw Arm (adds 1 p/s to melee damage and allows vertical wall climbing) +1 hp
Tentacle Arm (adds 1p/s to melee damage and 1 m/s to swimming/floating speed) +1 hp
blob body (adds an extra 10 hp, and maybe some suitably "jelly" features, like longer/faster swimming) +10 hp
bony body (adds an extra 15 hp, adds 1 m/s to running/swimming/floating speed) +15 hp
Prehensile Tail (adds 1m/s to the running/swimming/floating speed) +1 hp
Whipping Tail (adds 1 p/s to melee damage and 1m/s to running speed) +1 hp
Spitting Head (adds 1 p/s to short range damage) +1 hp
Biting Head (adds 1 p/s to melee damage) +1 hp
Example: Normal Large Earth-type animal:
9 parts: 1 biting head, 2 bony bodies, 2 Running Legs, 2 Jumping Legs, 2 prehensile tail
estimated capability: 6 m/s running/swimming/floating speeds, 6 m/s jumping power, 1 p/s melee damage, 41 hp (not overly horrific)
Example: Normal Large Earth-type squid:
10 parts: 1 biting head, 1 blob body, 8 Tentacle Arms
estimated capability: 9 m/s swimming/floating speed (fast!), 9 p/s melee damage (ouch!), 19 hp (not so tough)