Giant Horse Name
So naming him after Ganondorf’s horse, which doesn’t happen to have a canon name, is much more difficult then I’d expect. Now, Ganondorf’s horse is a black Gerudo Stallion never mentioned by name in any Zelda game or even the old TV series, but after doing some research, this is what I narrowed things down to for a potentially canon-influenced name:
Phantom- there is a Zelda Chess game where the Knight pieces are horses, for Link’s side are labeled as and resembling Epona, and on Ganon’s side labeled as Phantom and resembling his horse from OoT, which implies that Phantom is to be interpreted as an actual name and if true, is the only instance of a name ever given to ganon’s horse. This one is the most plausible.
Steed- literal interpretation, since Steed is capitalized in reference to “Ganondorf’s Steed” from various Nintendo-based sources, which could imply it as both a title and name.
Nightmare- theorized, mainly due to the OoT version of ganon’s horse resembling ganondorf himself, and alluding to the theory that his horse is one of his dark creations rather than an actual horse, much like Phantom Ganon. Plus Link first sees the pair in a nightmare at the beginning of OoT so yeah, if it fits it ships.
Galloughs- this one I read from an unconfirmed source that it is the actual actor name for ganon’s horse in OoT’s code. From what else I’ve read though, those who have dissected the game’s code found ganon’s horse simply referred to by generic description. Either way, Galloughs is a badass name and fits the 9-character limit for registering horses in BotW so I included it on my list.