I have the Gun Runners DLC for Fallout New Vegas and I am trying to complete one of the challenges that comes with the DLC, '(GRA) Overkill *':
Kill (20) Animals with any Fat Man or Fat Mines.
The problem is, I don't know what "Animals" means... I've tried shooting my Fat Man at a pack of Geckos. When I checked my challenge I didn't have any progress towards challenge completion (0/20). So then I tried to shoot a mini-nuke from my Fat Man at a pack of Bighorners. When I checked the challenge progress it still said 0/20.
What are considered as "Animals" for this challenge?