Humans have domesticated all manner of creatures during the course of their civilization - dogs, horses, several types of cattle, varieties of birds, and perhaps others too.
A prerequisite for an animal to be domesticated, obviously, is that there be some return from the animal voluntarily, or otherwise. For instance, milk from cows/buffalos.
What I'm looking for is an answer to the question what ... characteristics must an animal fulfil to be domesticated/trained. Docile is, perhaps, one characterstic. Amenable to training may be another. What other characteristics must be fulfilled before one would venture to consider training/domestication of another species?