* Millipede is a specific type of arthropod: It's not a general term like "insect."
* Antonyms are often about opposites: There's no creature that's the direct opposite of a millipede in terms of function, behavior, or anatomy.
Possible "opposite" concepts:
* Other arthropods: You could use a different type of arthropod like a spider, beetle, or butterfly. These are all different from millipedes, but not directly opposite.
* Creature without legs: Something like a snake or worm, though they are very different in structure and function.
* Something with no legs at all: A plant, rock, or object that has no movement.
Ultimately, the best antonym depends on the context of your usage.