Is it possible for creatures from different planes such as fey, celestials, fiends, genies, etc. be turned undead? There is no real ruling in the books on it.
I get that you can't turn an elemental undead, as they don't really have much of a corpse afterwards.
Specifically, a DM friend of mine wants to know if Ghoul Dryads and Vampire Pixies are possible.
Best Answer
There are a lot of ways to make a creature undead.
None of these methods will create a non-humanoid undead. However, as @keithcurtis points out, the Monster Manual includes a large number of undead that aren't made from humanoid creatures.
So clearly it is possible to have non-humanoid undead. So why are there no published methods to make one? Well, I have no proof, but I believe it's to ensure that players can only make undead using the standard statblocks. If, for example, Animate Dead worked on the corpse of any creature, there would either need to be zombie and skeleton statblocks for every creature, or some sort of standardized process for zombifying creatures. The first is pretty clearly unfeasible, and the second tends to be open to exploitation.
That's everything players have access to. However, for DMs, there is a large section on Creating Monsters in the DMG (page 273), including creating monsters by modifying existing monsters. For a Vampire Pixie, I'd recommend starting by applying the changes listed in the Player Characters as Vampires sidebar, Monster Manual page 295, and go from there. Other than their special abilities, which they probably wouldn't be able to use once they became Ghouls, Dryads are fairly similar to humanoids, so the Ghoul statblock shouldn't need much alteration to represent Dryad Ghouls.