Shapeshifters

From Dragon Ball - Wiki on Neoseeker

Bulma, Krillin and Gohan encountered two Shapeshifter beings on their way to Planet Namek. This tall, red/purple species can change their forms flawlessly, and even have the ability to change the appearance of their surroundings and home planet in the form of illusions.

Oolong and Puar once attended a Shapeshifters School on Earth, so they also have a basic sense of how to change their appearance, if only for a short time.