Ok listen I might have a schizo idea BUT naming them after clouds spooked me the fuck out:

If any of you heard about anon prophetic dream where Anon described a dream he had where he lived his whole life through what was basicly human harvest by aliens.

Invasion started with ships camouflaged as clouds that looked like anvils.

Now, we have nimbus variant and stratus variant, I think next will be Cumulus and the last one - Cumulonimbus that is in a shape of an anvil