I'd personally say that the armour rating for the Kelly armour is a bit high – it's more than a full set of gothic plate, which represented the pinnacle of (European) armour design. A bit much for something homemade and which didn't cover the arms or legs! I'd go with Medium/11/9pts (same as a steel breastplate – rationalizing that home-made kit with a helmet balances out professionally made armour without one) or Medium/12/16 (as SRC breastplate) at most.
But on the subject on which you actually wanted comments, I'm with you – horses aren't exactly common enough (though that may change with some of the supplement lists) to make it a worthwhile investment. It is flavourful, though. My instinct would be to make it something like...
Horse Thief (3pts)
If a character with this Talent contacts a riderless horse during his movement [N.B. this is covered in more depth in the supplement], he may claim it as his own by performing no other actions that turn. If on foot, he may mount the horse and ride it thereafter. If already mounted or not wanting to mount it, he will lead the horse behind him. Unless it escapes, that horse will remain in base-to-base contact with the Horse Thief. If the Horse Thief is engaged in hand-to-hand combat, Knocked Down or taken out, the horse escapes and follows its natural behaviour [again, the supplement explains all!]. At the end of the game, the Horse Thief may either keep the horse for himself (even if the Company List does not normally offer him that option), or sell it (in which case he gains a number of points equal to the mount's listed cost which can be spent in the same way as VPs acquired during a scenario).
3pts might be a bit cheap, given the possible rewards, but it does recognize the disadvantages that come with handling an unfamiliar horse in the middle of a combat environment!