She also has wings, which she can use to hover above the ground.
In her early concept, she did not have a visible mouth and her antlers were darker.
In Season 1, Penny had a straight, round shell similar to her father's, with noticeable holes where her arms and antlers came out of.
Similar to other characters in their Season 1 appearances, she also had a black shadow on the tops of her legs from her shell.
In Season 2, the design of Penny's shell is curved to resemble her sister's and her mother's. Also, the holes in the shell on her arms are missing, and the holes her antlers come out of are smaller.
In Season 3, the episode "The Shell" revealed that Penny underneath her shell is a shape-shifting, fairy-like creature.