Adult, or hatchling, depending on what you killed.
I did it last year with some horribly inbred dragons, ended up killing 3 adults and 1 hatchling and got 3 zombies out of it. I got lucky though. You can only kill 5 dragons too before you have to wait a few weeks to do it again, so you might have to kill something 2 months earlier to make sure your kills are up if you want to really have a shot at it... some people killed 5 and only got one zombie.
My only gripe with zombies is that you can only see them at night and that they keep moving on your scroll, lol. I love the sprite though.