What is the only animal with three legs?

According to Wikipedia, there is no such naturally occuring animal, though the kangaroo does sometimes use its tail in locomotion.

https://en.wikipedia.org/wiki/Tripedal

This is, of course, nonwithstanding birth defects, amputees, the Riddle of the Sphinx, or remarks inappropriate for funtrivia.

Tripedal (from the Latin tri = three + ped = foot) is the term used for (or would be used for) any animal that stands on three legs. The terms Bipedal and Quadrupedal are used more commonly when referring to animals that either walk on two legs (i.e. humans who walk upright) or animals such as dogs and cats who walk on four legs.

The terms triped, tripedal and tripedalism are rarely, if ever, used in a real scientific context, as there are no known naturally occurring three-legged animals on Earth. Although the movement of some Macropods such as kangaroos, which can alternate between resting their weight on their muscular tails and their two hind legs, may be an example of tripedal locomotion in animals.

https://en.wikipedia.org/wiki/Tripedal

There's that odd thing on the Manx arms - the triskelion. How it feeds I don't know, but it is regarded as being 'armless.'

https://en.wikipedia.org/wiki/Triskelion