Contrary to what the name suggests, they are not undead, but are normal people under the Cryptogriff spell. If someone has been hypnotized by a Cryptogriff Slug, their skin turns green and their eyes become red, matching the colour and pattern of the Crptogriff'. They lose their ability to speak, only making an eerie groaning noise similar to that of the undead. The biology of the hypnotized person will imitate the person before he or she was hypnotized. For example, a hypnotized strong person will create a strong zombie; a weak person will create a weak zombie; a fast person will create a fast zombie; a slow person will create a slow zombie and so forth. Being hypnotized seems to imply any one or thing that has the ability to reason can be hypnotized.
When someone becomes hypnotized they lose their free will and become a servant to the hypnotist. The Zombies never talk, just groan and when they move, they walk around in a stiff fashion but when acting on an order or fighting non-zombies they spring to life with great speed and strength, though they are still generally mindless. They have enough intelligence to understand orders. An example of this was when Mr. Saturday told them to get him more cushions and drinks, they knew what he was talking about, possibly through the connection between Mr. Saturday and his Cryptogriff, and the one between the Slug and the hypnotized people.
The only known way to free a hypnotized Slugterran is to reverse the ghouling process on the Cryptogriff Slug, turning it back into a Hypnogrif or by making Mr. Saturday's Control Staff/Blaster barrel spin in the opposite direction it is supposed to spin in, which can be achieved through the use of a Tormato Slug.