Though it is extremely dangerous, if a Hydra were domesticated (or held in captivity successfully), it does provide enough meat to feed virtually any size population. If you cut off one head, two more grow back immediately, so the Hydra only increases in total head count. Presumably, there is some amount of Hydra neck which also comes with each head, from which several good sized Hydra steaks can be cut. I'd imagine the meat would be a bit like crocodile; probably about halfway between chicken and fish. Obviously, Hydra farming is not for the faint of heart, but considering how much high quality meat could be harvested from a single Hydra in a very short time, it would certainly be worth it.
Griffons and Hippogriffs are obvious candidates for domesticated riding beasts. The ability to fly with a passenger is a pretty good reason to use them.
Medieval Europe and Japan both had lots of stories about domestic fey which would hide around a peasant hut and (in return for small amounts of food left out for it) would do work that benefited the peasants, like repairing furniture or buildings or reaping a field in the night. Certainly no mythology-derived economy would be complete without worker fey.
Dragons are always interesting, not so much as something that is likely to be domesticated, but potentially as something which might be partnered with. Most myths have dragons being as intelligent as humans, and certainly in Western myths, they could often belt out fire hotter than that produced by any forge. The natural outcome of that might be a dragon-powered blast furnace that can produce alloys unavailable anywhere else. Obviously, the dragon would have to benefit from this arrangement, so there would have to be a quid-pro-quo of something they want in return for a few hours blowing into a big stone tower.
Will-o-wisps or pixies kept in jars may be useful sources of light in a pre-electrical setting. Maybe they can be fed with honey, or something equally cheap (though the tales of will-o-wisps usually imply that they "feed" on the dead souls of poor fools who follow their lights into a bog and die).
There are many, many more possibilities...