While I can’t say for sure - that sounds like it is now behaving correctly. When mounts were first added to the game they could all fight properly. Then players complained that this meant PVP could now have two followers (follower plus mount) that both fought. So (after an initial change to make mounts completely irrelevant) it was changed so that mounts all run away and never fight. If the skeletal horse was able to attack, that was (almost certainly) an error when it was introduced. When the fixed it to being a mount instead of a follower, they almost certainly also fixed it so that it also no longer attacks, because otherwise it would be out of step with every other mount in the game.
(Attacks while mounted are a separate issue, since they just take the place of you striking from the saddle (or the rhino horn attack - if they even still have that). But running away should be the only behaviour a mount is capable of when it doesn’t have a player on its back. Personally I’d like to have mounts return to being able to defend themselves…)
That’s a shame - as with the rhino, that took the place of the player attacking from its back, so that couldn’t have been unbalancing anything. (I’ll have to check, but I have a feeling the rhino no longer has its mounted attack either.)
Well, that definitely doesn’t seem right - my understanding was all followers were supposed to have DBNO (except maybe the ones that were just purely created by sorcery). Might be worth a bug report?