Spoilers for Shadowbringers and Endwalker below
I think one of the most interesting mounts in the game to me is Grani, which comes with the Collector's Edition of Shadowbringers. Looking at its appearance, it's a pretty cool-looking creature. It shows up on the cover artwork for Shadowbringers Collector's Edition which I think is neat. It takes the appearance of a skeletal horse with ornate barding and a large horn protruding out of its head. Its name is a reference to a horse from Norse mythology that was said to be the steed of the hero Sigurd.
What I find really intriguing about Grani is the small bit of in-game lore surrounding it and its implications. Grani doesn't play much of a role in the game's story and might initially seem to some to be an ordinary, albeit cool-looking, mount; however, when you actually look into the information the game gives on it, things become a lot more interesting.
Grani's mount description is "Possessed of a strength of both mind and body that is unmatched in the First, as a mount, Grani has no peer on land nor in the heavens. It is said, however, the noble creature will only accept a rider who has pledged to restore shadow to a land bereft of night." Pretty standard description, right? When taking into account the only time Grani actually appears in the game's story, this is when things become deeper than they initially seem.
During a scene in Endwalker taking place in Elpis, Emet-Selch can be seen riding Grani. Not only does this reveal that Grani existed prior to the Sundering, it also implies that Grani was originally Emet-Selch's familiar; but if Emet-Selch is Grani's master, then why is it that in the current era, it will only accept a rider dedicated to restoring night to the First? Emet-Selch's motive involves making sure the First has an imbalance of Light so that another Rejoining can occur.
I think that the natural implication here is that Grani ended up leaving Emet-Selch. The reason for which this might be is unknown, but judging from the fact that it will only accept a rider who actively works against Emet-Selch's plans, maybe it took some kind of moral objection to what he was doing? Is that even possible? I'm not totally sure, I just thought it was a neat possibility to think about. What does everyone else think?