First the card:
Striped Cat
G
2/2
When Striped Cat is put into a graveyard from play, target opponent gains three life.
i think the card is okay as-is, but I would really like a cycle around this drawback, maybe with 2 additional creatures and an enchantment. The reason why I created this is because I think it would be interesting to have a drawback that makes your deck lose tempo on a temp-oriented card. Also, i like Berserk and Stompy in general, and i think it could have additional creatures. The problem is completing a cycle, because I think the drawback could be used on several cards in an expansion (also, it would be interesting to see several of these die because of Wrath).
This is what I thought could be a second one:
"Treefolk Dude"
1GG
4/5
When "Treefolk Dude" is put into a graveyard from play, target opponent gains five life.
Five life is a lot, but on the other hand, this is close-to-immune to burn. I don't know whether this one is too good, especially with a possible 2nd turn Treefolk Dude start.
Adding a third [and a fourth one, thanks Matt!) one:
Current Wordings:
Striped Cat

Creature-Cat
2/1
When Striped Cat leaves play , target opponent gains three life. Striped Centaur

Creature- Centaur
3/2
When Striped Centaur leaves play , target opponent gains four life Striped Murlodant

Creature- Beast
4/4
When Striped Murlodant leaves play, target opponent gains five life.
Mutant Beast
Creature- Beast
6/5
When Mutant Beast leaves play, target opponent gains 8 life