So, I’ve thought about this, and the best way to incorporate Roach in a ponified Witcher universe would be as a female Witcher and a love interest for Geralt. After all, she’s been his faithful companion for a while, and while she can’t be his trusty steed, he can still ride her…
Geralt pony: “Why do kelpies have to disguise themselves as one of our kind so often? This job is hard enough as it is without the incurred guilt or the ignorant townsfolk screaming bloody murder.”