What happens when you banish an elemental creature that was formed/born on the Material Plane?

Question

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.

Answer 1

Most elementals are native to the elemental planes

The Introduction to the Monster Manual (p. 6) and the corresponding portion of the basic rules mention in their description of the "elemental" creature type:

Elementals are creatures native to the elemental planes.

For these elementals, even when they are formed on the Material Plane, they are still considered natives of the respective plane. For example, the "Azer" section in the Monster Manual (p. 22) begins:

Natives of the Elemental Plane of Fire, azers are master crafters, expert miners, and sworn foes of the efreet.

...and the "Elementals" section (p. 123) reads:

Though little more than animated energy on their own planes of existence, they can be called on by spellcasters and powerful beings to take shape and perform tasks.

...implying that spellcasters (most of the story-important ones of which live in the material plane) are bringing them from a "native plane" elsewhere.

There are exceptions

While the general case for elementals is that they are native to the elemental planes, there are some specific examples of elementals that are from and live exclusively within the Material. One such example is the chwinga (found in the Tomb of Annihilation adventure module, p. 217):

A chwinga is a tiny elemental spirit that lives in plants, rocks, and rivers far from civilization.

None of the elemental planes have plants and rivers:

[In the elemental planes], the elements exist in their purest form--great expanses of solid earth, blazing fire, crystal-clear water, and unsullied air

...so this must be a reference to some other plane (likely the Material since that is where they are found in the module).

In the case of your GM's elementals, he will have to decide whether the elementals being formed on the Material are their own type of elemental (and thereby could be native to the Material), or just forming based on the elementals that are native to the elemental planes.

For a real-world analogy, if you brought an iguana egg to the desert and hatched it, the iguana would not then be native to the desert (since iguana are native to rainforests), but if it is among the first of its kind, it could only be native to that region.

Answer 2

This is Homebrew, so it's up to the DM

This represents a change to the monster and determining how that change interacts with existing mechanics is up to the DM.

There is no right answer here from the community because we didn't create it and it really can interact in any way because of that. Whomever created it can decide how they want spells like banishment to react with it and come up with reasoning for that interaction.

Logic supporting either option can be inferred from the monster descriptions, so it really is up to the DM.