I was under the impression they were fairly small animals - the screenshots of the tloke sighting made them look much too small for even dwarves to use for riding. Though I suppose there could be multiple kinds of tloke, and some are bigger than others.
Being herbivores doesn't necessarily mean a lot, even if they're good natured herbivores. IRL there are numerous animals that are tamable and even breedable, but probably won't ever be really domesticated (zebras, for example), since things like the animal's social structure, attention span or tendency to spook cause problems. Tlokes could always be one of those things that people keep around because they're cute, not because they're useful.