Mod adds a spell tome in the companions guild living area
Spell 'should' only turn followers into horses
Spell will only work on one companion at a time even with UFO etc
Follower is knocked down after transformation to re-initialize AI packages
Player will be forced to dismount 5 seconds before the follower transforms back
:)

Should work with everything from vilja to sofia to Dhalia and Allune

Russian translation  http://www.nexusmods.com/skyrim/mods/56792/?