Asymmetric creatures

See also: mutants, body horror

Generally speaking, human understanding of aesthetics is first and foremost driven by symmetry. Minor flaws can and are ignored, but the moment a certain threshold is crossed, any living beings start looking ugly as sin instead of just non-human. Even monsters can be aesthetically pleasing as long as they maintain symmetric appearance. Note that even asymmetric creatures can still be aesthetically good, though for them it's rarer. And for sexualized non-humans asymmetric appearance is hardly ever present.

Some exceptions to this exist, though they're mostly known in some manner or other, though most aren't worth this tag. Heterochromia (eyes are of different colors), a mutated hand/arm, mutilated one half of the face, and so on.

A good example would be a character with three arms on one side and none on the other (or maybe a tentacle instead).