Add one or more parts, not usually found on the being in question, such as: third eye (back of head), feelers (radiation sensitive), antennae (light sensitive), pincers, fur coat, feathers, radiation absorbing organ, and so on.
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FOURTH EDITION:
The mutant grows body parts not normally found on their species. The character gets as many as these parts as is reasonable, one tail, a pair of pincer claws, etc. Animal and plant characters can get true arms with hands, snakes can get real legs , etc. Whatever the mutant acquires, it must be usable and beneficial. A second head means only single brain, but there are two sets of sensory organs (eyes, ears, etc). Roll a d10 on the table below. If the character already has that body part, they can choose to get a second set of them or reroll. Altered humans suffer a -2 penalty on Robot Recognition for having the new body part mutation.
Roll Part
1 Tentacles
2 Trunk
3 Extra eyes
4 Tail
5 Second Head
6 Claws or Fangs
7 Crab-like pincers
8 Arms or Legs
9 True Hands
10 Kangaroo-like Pouch