While researching this question about polymorphing a paladin's steed, I had a realization. Polymorph specifies a creature you can see as a target, and as answered here, "creature" essentially refers to every monster/PC/NPC that you might come across. This means that polymorph can change a spirit, ghost, or other incorporeal thing into a beast. Am I understanding this correctly that, as written, you can polymorph a ghost into a living thing? Am I missing something?
That is my understanding.
That is supported in the DMG and it list creature types it opens with
A monster's type speaks to its fundamental nature. Certain spells, magic items, class features, and other effects in the game interact in special ways with creatures of a particular type.
Among the list of creatures are fiends, celestials, fey and undead. These are the types that apply to most "ghostly" creatures -- especially that last one.
Ghosts, Banshees, Revenants are all undead creatures. It even speaks of a lot of fey and demons as "fey spirits" or "demonic spirits." (Even the Rakshasa is spoken of as an "evil spirit" -- though it does mention "in Mortal Flesh".)
If this wasn't the case a lot of spells could never have been used on the above types of creatures. You could attack them either, as RAW you can attack objects and creatures. If a spirit was something else, besides a creature, those types which have ghostly things, would be immune to all damage. While some have communities and/or resistances because of their ghostly nature, the rule system still calls them all creatures.
Takes on the form
Moreover, when you summon the steed, it takes on the form and stats of the creature summoned -- which would include the creature type. So, even if you couldn't use polymorph on a Ghost, which you can by above, you'd still be able to polymorph the horse you summon with find steed.
Incorporeal in 5e
The ghost isn't completely incorporeal (like we think of Ghosts) in D&D 5e. It can be cut by blade or shot by arrow. It's incorporeal nature in 5e is modeled by resistance to non-magical weapons, a movement ability, and the ability to slip into and out of the ethereal plane as an action. So, you can touch the physical form of a ghost, if it is on the prime material.