This was pointed out on tumblr here.
[link


They come from Mexican Myth (TW season four will revolve around Mexico)

From the Grimm Wiki, these creatures : When woged, Balam grow 모피 over their head and body, but like many Wesen their hair is unaffected. They displays distinct stripe-like spots similar to those of a jaguar 또는 a leopard. The color of their 모피 seem to change depending on the brightness of their surrounding. It appears purple in a dark environment and light grey when in the middle of the day. Like fellow Wesen Blutbaden 또는 Drang-Zorn, they are able to concentrate their woge around their eyes, making them amber and appear to glow. They also gain two sharp jaguar-like fangs.

So could this be what Kate is?