We know almost nothing about them

But 'catpeople' in varrying degrees is how fandom interprets the Felinid strain of abhumans, which have never been seen in the video games or tabletop.

Please, if you are reading this, tell me what you imagine a 40k cat-person is like

probably medium deformed and awful

yeah, that would scan with the overall setting

instead of cute anime fangs you get teeth that need to be extracted and non-functional catears

Ahhh, the Puma Sisters That is a possibility

Separating Felinid Fact From Felinid Fiction - Codex...

All that is known