Rare Large Mob Variants

To add more surprise in minecraft, or a collector item for people who want to hunt, would be rare large mobs. When a mob spawns, it could have a chance to become huge-ish, or when a mob grows up. Upon the large mob's death, it could drop more loot than the normal mob, more EXP, and a mob head for the people who don't care about collecting, and the mobs could also do more damage than usual if they did damage in the first place.