The mayor of a Texas town was killed by his own donkey. I don't hear of many donkey attacks; the ones I do hear about are usually a matter of territorial defense, as donkeys don't like strangers in their pens. For that reason, some farmers use them as guards for llamas and other livestock.
Territorial defense doesn't seem to be the case here, however. Authorities explained it thus:
Texas mayor killed by huge pet donkey — RT:
"“They can become very aggressive, very mean, sometimes triggered by a female in heat,” said Atoscosa County Chief Deputy David Soward. “We’ll probably never know what triggered it, but it was evident that this particular donkey was involved, based on the evidence at the scene and what we saw on this donkey.”"
I've often been told that donkeys are more intelligent and less temperamental than horses.
Update: Dee found this interesting video about a donkey guarding livestock against a cougar: