A Slug is a Creature that has no legs whatsoever. Because they lack legs, Slugs will often have a hard time moving, and will simply slither around, making them easy targets for predators, and they will also be unable to dance due to a lack of feet. However, adding wings to a slug will allow it a limited form of flight, and may prove crucial in a predator/prey confrontation. Movement Problems can be remedied with the Creature Part Icky-osaur (see Details), which provides Level 1 speed.
An unfortunate glitch causes many players whose creatures lack feet to crash the game when they die in the creature stage. If you are trying to get this achievement, it is recommended that you save often.