WMG / Star Trek Voyager S 4 E 4 Nemesis

The Vori actually resemble the Kradin, or at least appear very similar.

While it's suggested on the recap page that the Vori's human appearance is part of the brainwashing program (likely the psychotropic aspects, specifically) in order to make the target more receptive, taking this a step further would have their actual appearance be anything from identical to the Kradin, similar to them, or at least equally "beast"-like to a human's POV to underscore that the viewer can't know which side is right (because neither might be), which side is bad (because they both are), or simply that there is no Black-and-White Morality here. The reason this wasn't shown, aside from saving on makeup costs and helping Chakotay/the viewer identify more with the Vori, was to keep from being Anvilicious.