Chameleoleech

Chameleoleeches are identical to ordinary Giant Leeches, except that they have the ability to mimic the appearance of both living creatures and inanimate objects - a chest full of treasure, a person, a monster, and so on. The creature seems able to respond to its environment in some unknown way, adopting whatever form would make it seem most inconspicuous. Since the mimicry is purely illusory (affecting visual senses only), the change in appearance is instantaneous. Often, several of the creatures may lair together, forming a multiple illusion that is especially dangerous for the unwary. Note that a Chameleoleech cannot reproduce spell effects, speech, or attacks. Illusions are limited in size to a minimum of one foot and a maximum of six.