Whats more difficult than a human (2 legged) walk cycle? You're right, it's a 4 legged walk cycle. The legs are drawn differently to show near and far legs. The whole idea with these line tests is to get the animation right without spending long on making the drawing beautiful, then you clean it up later once everything is moving as it should.

Interesting fact - I've done a natural animal walk cycle here, but for comedy effect you can also use two human walk cycles, like a pantomime horse. Pluto is drawn like that (apparently, I haven't checked).