Gastropods like slugs and snails walk, by definition on a single muscular foot, however a foot is not necessarily a leg, so they don't really count.
Others that seem eligible are dolphin and so on. Although technically speaking they have three limbs but four legs, two of them are no longer visible and two have been modified to become flippers (although they are all derivatives of the pentadactyl limb and contain the same number of bones as almost any other animal's legs). The tail is just that, a tail, which is an extension of the spine so that isn't a leg either.
With reference to the starfish idea it's not a bad idea, but as Smiffy65 rightly says they are arms not legs. But, just for the record they do actually have feet as well. Hundreds of tube like thingies on the underside known as podia. Sadly though, regardless of the numbers of them, these are still feet and not legs.
In short my opinion is that there is no known, natural species of animal which has an odd number of legs (amputation aside). But that's not to say we won't discover one in the future.
Besides that there really ought to be an animal called a monopede (like a millipede except with one foot), or more correctly a monopod but that name's already taken. It would be funny but it probably wouldn't last long according to Neo-Darwinian Natural Selection.