Apparently people who don't monitor their spending; I've seen a number of rocks with eyes glued to them being sold in novelty shops.

[But, seriously. Who has the money for that when you can just make one-- if you even wanted one.]

I think perhaps a small sparrow might be best. The most damage it can be trained to do is ruining someone's shirt.

[Because you know Donut would probably train a dog to follow him into battle...]