We could try to make this clear by claiming the case that Torrent is more horse than goat, or at the very least, a mix of both: watching footage of goat walks their legs are quite straight. Because Torrent's legs appear more horse-like than Elden Ring Runes... caprine's I'm not entirely happy with that explanation. Particularly since there are horses that aren't goats in this game that are enemy mounts, and they tend to be the same.
Although it is visually jarring it is, at the very least, an element of truth to it: goat legs may appear odd however they are functional. Playing the game for a longer period has helped me understand that this choice is understandable from a game perspective in a game such as Elden Ring, where precision and timing are crucial to defeat any opponent, your horse's movements being inconsistent could cause you to be prevented from using it in combat.Meaning I understand the reason why smooth movement is favored over the realism of horses.
I also would think that in Elden Ring's design process, someone thought, "Maybe the player needs horses," and somebody else added, "Not fucked up enough that it should be a goat with cheap Elden Ring Runes horse legs," everyone nodded.