I left out the part where steeds, being int 6+ and who understand language, can attune magic items. You need an item that can be used by the steed, though. Pegasi and dire wolves/tigers are pretty limited in options. Hats and helms are generally a no. Also things that require command words are out.
Mantles and cloaks can be a saddle blanket and circlets, periapts & necklaces can work. Our DM agreed that griffin forelimbs are sufficiently "fingers" and "arms" they could wear bracers and rings (especially when said items were acquired from Large+ creatures and already fit, so no debate about what Attunement did for resizing). Tbh, bracers could probably work on many steeds as a horse's cannon can easily be more slender than many human forearms.
Those weird items that only get attuned until something better shows up? Yeah, steeds get those. Ring of the ram? Ok. Circlet of blasting? Yeah. Rings of water breathing and swimming on an in-land campaign? Done and done. And then, at some unexpected moment, those items show up and the steed gets a moment. A shadow gets zapped by a circlet. An enemy is punted over a cliff by the ring ram. A sahuagan steals an item and dives into the water, only to be hauled out by a griffin like an eagle catching a salmon. Its a bit of happiness, right there. Nice for the allies to have their own glory.
Lastly, at tier4, true polymorph can turn them into humanoids, which has entertaining RP opportunities. Although the combat value of a pegasus TPd into a different colored pegasus is pretty great. The baddies beat down the paladin's steed who then magically changes color and almost all their injuries are healed. "Oh no, you've awakened its final form!" (Doesn't work as well for most of the other forms as they get turned into beast-intellects)
