User blog comment:Kurare/Yoruichi Shihōin's Zanpakutō theory/@comment-24113610-20140821173926

I highly doubt Yoruichi's shikai is the ability to turn into a small black cat with little to no power at all except for speed, which isn't nearly as close to her human form's speed.

I think it's just an ability from the Shihoin clan. We've never seen any other members of her clan so there's no way to confirm this yet but that's more likely than turning into a powerless cat being her shikai. I do think it's odd that we've never seen her with a zanpakuto at all though.