I have noticed (and I guess the Wiki as well) that the pets that you could ride on cannot be rode on anymore. Not sure why though. Is there a reason behind this?
The Wiki still has the "Ridable?" category to distinguish the mountable pets from the non-mounts. So probably this might be a temporary "fix"? But that's just my speculation. Perhaps pets are now permanently by your side and not between your legs.
Looks like the Cowboys got a big nerf this month. One being the grenade charging, and another being the incapability of mounting a pet.