A few spells mention using spirits, specifically fey, fiend or celestial spirits, usually regarding summoning them to take the form of another creature. A few examples are listed below:
Conjure animals (PHB, p. 225):
You summon fey spirits that take the form of beasts and appear in unoccupied spaces you can see within range. … Each beast is also considered fey, …
Find familiar (PHB, p. 240):
You gain the service of a familiar, a spirit that takes the form of an animal … the familiar has the statistics of the chosen form, though it is a celestial, fey or fiend (your choice) instead of a beast.
Find steed (PHB, p. 240):
You summon a spirit that assumes the form of an unusually intelligent, strong, and loyal steed … the steed has the statistics of the chosen form, though it is a celestial, fey or fiend (your choice) instead of its normal type.
Some of these I am identifying as fiendish or celestial spirits by inference, although conjure animals explicitly says "fey spirits".
My question is, what exactly is a fey spirit? Or a fiend or celestial one? (If this question is considered too broad by asking about all three, I'll happy reduce the scope to just "what is a fey spirit", but I don't see these as being too different to make this too broad, personally).
The only way I can make sense of this is to assume that this is referring to a fey being that has died, so literally the ghost of a fey. But then why can't some of these spells use an elemental's spirit, or even a humanoid's spirit? But even then, wouldn't these spirits be undead type, rather than fey or whatever, if they were the spirits of dead creatures? Shouldn't spirits of dead fey have gone to some kind of afterlife; how come relatively low level magic can summon them? Maybe it isn't literally a dead fey, but then what is a fey spirit?
Note that I don't expect each of the questions in the above stream of non-bold questions to be answered, they are just included to try to give some insight into my train of thought/confusion.
Also note that, although I'm asking "why" this or that, this isn't a designer-reasons question, this is a lore question, since I'm interested in the in-universe justification for things, not designer reasons. I don't care why, in the read world, Jeremy Crawford or Gary Gygax or whoever else from however far back this goes decided that this is the case, only how this makes sense in-universe. What, from a lore perspective, is a fey (or fiend or celestial) spirit compared to an actual fey (or fiend or celestial) creature?
If a setting is necessary to answer this question, let's assume Forgotten Realms, but I don't actually care about a specific setting. Given that this is a lore question, I'm also happy for answers to include information from previous editions, but I'm primarily interested in information from 5e.
Best Answer
Your emphasis is on fey, but it’s actually easier to start with fiends and celestials.
Also, this is going to necessarily be based on the history of D&D, because 5e hasn’t really gone into a lot of details about this sort of thing. That said, everything I claim here is consistent with 5e, including the things that they have changed.
OK, so then, fiends and celestials are what previous editions of D&D called “Outsiders,” non-mortal creatures from other planes of existence. They are related to “Elementals,” and share many similarities with them—one could think of Outsiders as being the “Elementals” of non-elemental things, most notably belief. That is, where a fire elemental is a being made of fire, “fire incarnate,” a fiend is a being made of evil, “evil incarnate.” Celestials likewise but good. In the wider multiverse of D&D, most thoroughly described in the Planescape setting of 2e and 3e, belief in alignments is potent stuff, giving rise to entire planes of existence (the heavens and the hells and so on), which are made out of solid belief in that alignment. Fiends and celestials are made out of that same solid belief as the plane they originate upon—and since those planes of belief are known as the “Outer Planes,” they are known as Outsiders (for the record, the “Inner Planes” would be the elemental ones, but no one calls elementals “Insiders”).
One of the key things about Outsiders and Elementals both is that they do not exhibit “dualism,” the concept of a soul and a body as separate entities. For an Outsider or Elemental, their soul is their body and vice-versa. This allows their body to radically change in tune with changes to their soul—since those are the same. For instance, a marilith, a six-armed demon with the lower body of a snake and the upper body of a woman, could become a balor, a hulking, furry brute with horns, cloven hooves, and enormous wings.
A balor and a marilith, as depicted in the 3.5e Monster Manual.
Likewise, they could be bound to other forms, say objects. They could inhabit other creatures, possessing them. And so on. This is how fiendish or celestial “spirits” can be used as a familiar or steed.
How do Fey work into this? In previous editions, they didn’t; this use of “fey spirits” is new to 5e. However, some Fey creatures were incorporeal spirits—no body to speak of. Unlike, say, a satyr or dryad, some fey were only spirits—making them more like Outsiders or Elementals. And the Fey were largely associated not with the elemental planes, or the planes of belief, but with the material plane1—the plane of mortals. They are, in some ways, analogous to Outsiders and Elementals for the material plane. And this is exactly how 5e uses them: where in previous editions, a hag was a “native outsider,” that is an Outsider of the material plane, in 5e hags are classified as Fey creatures instead.
So 5e changed Fey to be more like Outsiders, and so it would seem this included the greater malleability of Outsiders, to allow them to be bound into conventional animal forms.