Mantenna always looked awkward and goofy, But I did liked that his eyes would pop out of his head... It was creepy.

That and his multiple limbs made me think of him as an insect/crustacean kind of creature, of those, non is weirder than the Mantis Shrimp (who can see colors humans cannot!), So his eyes are modelled after a Mantis Shrimp's and I envisioned him being able to see several dimensions at once, his eyes radiating different energies to harm his foes and generally making everyone around him uncomfortable...