Background: In one of our campaigns we have quite a few Elementals that are forming in the material plane, as opposed to forming on an elemental plane. The elementals are otherwise identical to the ones appearing in the Monster Manual.
We are coming up to the levels where some of us can choose the 4th-level spell Banishment as a spell. I was wondering if casting Banishment on an elemental formed on the material plane would a) banish it to an elemental plane and not return or b) be only temporarily banished to a demiplane and appear back onto the material plane when the spell ends.
The way I see it is that it would have to do with the interpretation of what is "native to the plane" and "home plane" in this case.
The description of the Banishment spell (PHB, p. 217) says:
You attempt to send one creature that you can see within range to another plane of existence. The target must succeed on a Charisma saving throw or be banished.
If the target is native to the plane of existence you’re on, you banish the target to a harmless demiplane. While there, the target is incapacitated. The target remains there until the spell ends, at which point the target reappears in the space it left or in the nearest unoccupied space if that space is occupied.
If the target is native to a different plane of existence than the one you’re on, the target is banished with a faint popping noise, returning to its home plane. [...]
I suspect it would banish these home-grown elemental creatures to a demiplane in the case of our campaign; unless we were on another plane, in which case it would be banished to the material plane as this is its "home plane".
There is this question which gives an answer of sorts: What determines a creature's native plane for the Banishment spell?, but I would like a more concrete answer if possible to the scenario I present above.