I'll put this mostly into comments instead of it being a wall of text. I've scavenged these from a bunch of different articles and videos, plus some help from Kolanaki. Some of it is on the wikis now
Spectral steed regalia
Horse armour, but FromSoft
Tree sentinel regalia: A smaller version of the solemn gold armor that adorns the warhorses of the Tree Sentinels who serve the Erdtree. One of several liveries once prepared for spectral steeds, which allows Torrent, through grace, to change his appearance. Though a spectral steed's form will change when vested in this garb, their heart will remain the same.
Carian silver regalia The richly ornamented white silver armor of the Carian Royal line is a one-of-a-kind item made for a certain spectral steed, once forged in the lands of the Three Sisters. It is through grace that the spectral steed Torrent's appearance may be changed. Though a spectral steed's form will change when vested in this garb, their heart will remain the same.
Funereal night regalia: A smaller version of the sable attire used to adorn and conceal the warhorses of the Night's Cavalry. One of several liveries once prepared for spectral steeds, which allows Torrent, through grace, to change his appearance. Though a spectral steed's form will change when vested in this garb, their heart will remain the same.
The Carian one seems to imply that it was Torrent's regalia specifically, and reinforces the connection between Ranni and Torrent. I think it's fairly safe to say that Torrent was once Ranni's steed, presumably lost when Ranni burned herself
The Funereal one confirms, to me, that the Night's Cavalry ride spectral steeds of their own. They can re-summon their steeds, after all