I've had a few mob mod ideas in the past. Here are ideas I've thought of for mobs:
Mob 1:
LeCriofhead: Skin here:
http://www.planetminecraft.com/skin/lecriofhead/Drops: Lapiz Lazuli (100%) Cake (5%) Raw Fish (35%)
Health: 12 Hearts Mr. Mittens has the same hearts as an ocelot
Behavior: A Neutral mob that wonders around Beach and Plain biomes. He carries around a Lapiz Lazuli and always drops it. The raw fish are for his cat, Mr. Mittens, who follows him around and attacks the player if he/she attacks LeCriofhead or Mr. Mittens himself. Mr. Mittens is a black tamed ocelot.
Damage: LeCriofhead does 1 heart Mr.Mittens does 3 Hearts.
Mob 2:
Puppet: I have texture on my desktop
Drops: Sticks (50%), wood (95%) and wooden planks (20%)
Health: 5 hearts
Behavior: A Hostile mob that is as fast as a Baby Villager (pretty fast). Always in packs of 3-5 Puppets. Same model as Skeleton, only half size and arms down. Appear at day, despawn at night and live in Jungle, Plains and Forest biomes.
Damage: 2 Hearts
I have more, but I won't post them at the moment...