The mount summoned by the 4th level Paladin spell find greater steed has this interesting ability:
While mounted on it, you can make any spell you cast that targets only you also target the mount.
The 6th level wizard spell Tenser’s tranformation grants weapon proficiencies;
You have proficiency with all armor, shields, simple weapons, and martial weapons.
After casting Tenser’s transformation while riding my found greater steed, technically my mount would have proficiency with all simple and martial weapons.
Obviously the primary issue here is going to be the question of “appropriate anatomy” – can magic that makes the uncoordinated wizard with no weapon skills a martial juggernaut also give my griffon the basic ability to firmly grasp a short sword? Are any of the mounts listed1 able to make use of any of the weapon proficiencies granted by Tenser’s transformation?
1 a griffon, a pegasus, a peryton, a dire wolf, a rhinoceros, or a saber-toothed tiger.