Striders. We've all seen them, and (nearly) all of us love them. I mean, how couldn't you? They're adorable, they're useful and they're something else idk I couldn't think of a 3rd point. Anyways, some of them weren't so lucky and didn't end up in the warm lava oceans. These unlucky few ended up looking quite different from the striders we're used to, and evolved to do nothing other than walk back and forwards across soulsand valleys, carrying Wither Skeletons around.
Anyways I can't think of any more lore so yeah, here you go.