Forum

In the core we have Wolfkin. In Bestiary we have Cat People, Frog People, Lizard People, Tree Kin, Ant People, Beetle Kin, and Spider Kin.

To be consistent, they all should follow the same naming conventions.

So either
Wolfkin
Catkin
Frogkin
Lizardkin
Treekin
Antkin
Beetlekin
Spiderkin

or
Wolf kin
Cat kin
Frog kin
Lizard kin
Tree kin
Ant kin
Beetle kin
Spider kin

Or give each a specific name because we have humans, dwarfs, elves, etc. and we don't call the mallard duckkin.

Or do we differentiate naming conventions from playable kin from non-playable kin?

Yes, I completely agree. I like your first list the most and is likely what I'll be calling them.

To be consistent, they all should follow the same naming conventions.

They should not. We are not that consistent in the real world.

...we don't call the mallard duckkin.

Nope, just Ducks (or Anatanthropus donaldi) if we're getting back to our RuneQuest roots.