I can't find in the books the definition of “native”, only that a creature type can define its native plane:
Elementals are creatures native to the elemental planes (MM, p. 6)
Celestials ands fiends have a similar line.
By RAW its seems like your native plane isn't determined by what plane you were born on, since a succubus/incubus born on the material plane is native to the lower planes, just as a cambion is.
So I was wondering: If you banish a druid in fire elemental form, does he travel to the plane of fire? If a Slaad* is born from an humanoid body on the material plane, is it native to the material plane?
* Note that slaads are listed as aberrations.
Best Answer
Sadly the rules are, if not silent, very quiet on this issue. In 3.5e this was handled by the Extraplanar subtype that creatures acquired when they traveled between planes and the rule that any creature that was not Extraplanar defaulted to being a native of the material plane. 5e is a brave new world though so we can't just use those rules.
Native, or indigenous, species are species that exist in a given ecosystem without being introduced by human activity. This makes sense in our world where the term is generally used when distinguishing between parts of an ecosystem disrupted by human presence and parts that we haven't gotten to yet.
So how about D&D? In D&D the term, "native to this plane of existence", could mean, "is meant to be on this plane of existence and is only found elsewhere due to intelligent intervention". Let's look at how that works out.
Using this definition leads to some tricky questions but generally works. Importantly it seems to match the intention of the spell to send creatures back where they belong.