An easy twist to add to the game is what I like to call Natural Selection. Basically, if somebody creates a natural creature set (i.e. the front, middle, and back cards of a single creature), that creature is unbeatable. All of the other players have to defend, if possible, and the scavenge rule still applies, but regardless of total value, a "natural" creature automatically wins and becomes a "Champion" creature.
It's not often that one player gets all three cards for a "natural" creature, so this is a nice little bonus, especially for the inherently weaker creatures.
Variants on the variant:
Rather than being invincible, a "natural" creature gets a 10 point bonus to it's strength. Or, feel free to make the bonus whatever you like.
A "natural" POISONOUS animal not only kills the attacker, but also becomes a "Champion" creature immediately (i.e. it doesn't get discarded per the usual poisonous creature defense rule).