Hello, new problem! XD
Some months ago I had installed a good number of mods, included the one called "Convenient horses". It was really useful for the features it introduced, but after some time the horses I owned have become not rideable, Frost included. I'm sure I have messed up a lot skyrim while installing and uninstalling the mods! Now I've returned completely to Vanilla (+official dlcs), but the problem remains, even after removing the entire game and reinstalling it.