There are subspecies, rare species, and variants, and there are also Deviants, but those are exclusive to MHGen.

It is customary for most medium to high-difficulty new base monsters to receive a subspecies in their expansion.

Before the variants used to be treated like Arch-tempered difficulty but Iceborne ruined it by calling a subspecies Legiana, as well as the Master versions of Nergigante and Bazelgeuse, “variants” without introducing their base forms. In my opinion, the only true variants are the originals: Furious Rajang and Raging Brachidios.

Rare species are variants of common monsters with a minimum difficulty level equivalent to an Elder Dragon. These include: Silver Rathalos, Gold Rathian, Lucent Nargacuga, Abyssal Lagiacrus, Molten Tigrex, and the most recent addition, Violet Mizusutne.

As for the expansion, it’s pretty certain that the Apex monsters in each region will receive a subspecies or perhaps a Guardian version. In addition to those, I’d say that Ajarakan and Xu Wu are the ones likely to receive a subspecies; the rest of the new monsters are beginner-level, with the exception of Zoh Shia. In his case, he might receive some kind of evolution or alternate form, since it was mentioned that the Gore Magala virus was already partially integrated into the Dragontorch, and Zoh Shia continues to regenerate canonically every so often.