Nyx has a werepanther form she can use if she's got enough energy stored up! (Read as: I'm a sucker for winged big cats, and I couldn't resist turning Nyx into one, at least part of the time.)
While in panther form, she typically has small feathered wings and a faintly glowing halo around her head, similar to the light around a full moon. This ref shows the wings about as big and the halo about as bright as they get - usually the wings are smaller and the halo dimmer. As always though, under the right circumstances they can take on a more dramatic appearance! A good example would be eclipses - there's a lot more ambient energy around those times, which means extra cool effects for someone as tied to celestial magic as Nyx is.
Her tattoos are also visible more often in panther form, since the ink uses magic show up through fur and her wereform gives off a lot of it. Sometimes they show up in her little cat form too, but it's more of a conscious effort on her part then.

In panther form, Nyx is usually about a foot taller than her little cat self, so about 6 and a half feet. Depending on the magic available to her she can get much bigger, though! She's usually a little shorter than Zephyr in this form and about on par with her partner Juniper. You can see her compared to Zephyr here.
There are also more general worldbuilding notes on my tumblr, which may end up posted here at some point.