This was pointed out on tumblr here.
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They come from Mexican Myth (TW season four will revolve around Mexico)

From the Grimm Wiki, these creatures : When woged, Balam grow fourrure 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 ou a leopard. The color of their fourrure 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 ou 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?