Centaurs have the Fey trait (Guildmaster's Guide to Ravnica, pg. 16):
Fey. Your creature type is fey, rather than humanoid.
What does this mean in terms of gameplay? I know that certain spells, such as hold person, require a humanoid target, so a centaur wouldn't be affected by that spell. But what about any implicit mechanics?
In the intro to the Monster Manual (pg. 6) and the corresponding section of the basic rules, it says under "Type":
The game includes the following monster types, which have no rules of their own.
So does this mean that the only consequence of them having the Fey trait is that they cannot be affected by spells and effects that target humanoids, and can be affected by spells and traits that target fey, and that's it?
For context, the reason I'm asking is that I want to give the playable elven subrace Eladrin the Fey trait as well (to make them consistent with the "monster" Eladrin stats, where they are listed as fey), and want to know the mechanical consequences of doing so.
Best Answer
Many small consequences, some with class features, others with spells.
Mechanically speaking, yes, the only consequence of them having the Fey trait is that they cannot be affected by spells and effects that target humanoids, and can be affected by spells and traits that target fey. Humanoid-targeting features are a lot to list (some that come to mind are hold/charm/dominate person, lycanthropy, the warlock's Gaze of Two Minds eldritch invocation, calm emotions, crown of madness, or simulacrum), but below is a list of all the fey-related features.
Circle of the Land druids can't be charmed by Centaurs.
Oath of the Ancients paladins can turn the faithless:
They can be a Ranger's Favored Enemy (without having to pick two specific races of humanoid) and are revealed by their Primeval Awareness:
Arcana Domain clerics' Arcane Abjuration Channel Divinity option can affect fey:
You might be detected by commune with nature:
You are detected by detect evil and good:
You are resisted by someone affected with dispel evil and good or protection from evil and good:
Forbiddance might harm you:
Hallow stops you from entering, unless the caster makes an exception:
Magic circle can affect you:
You can be bound to serve someone with planar binding:
You can be left out of a temple of the gods:
Some spells force a fey back to its homeplane, but I don't think Centaurs have a different home plane, so ignore those. With a home-brewy DM, you might be able to conjure a centaur with conjure fey, or conjure woodland beings.