I can't replicate the bug 100%, but sometimes when my pet is auto-summoned after a dismount, it will be missing its focus bar. The game registers the pet as having some focus (it will use a few specials -- I tested with a worm and it would use acid spit and growl) but it won't use most of them for lack of focus (thunterstomp, for example).

The problem can be solved by dismissing the pet and re-summoning it. The focus bar reappears, and the pet uses its abilities normally.

This seems to happen most often after dismounting from argent tournament mounts, or when summoning a pet for the first time after leaving the argent tournament area.