Negato = onyx + henna + wolfgrey + slyboots
(In other words, if you breed a cat that has all four of those traits, it will turn into Negato the Ninja Fancy.)
But here’s the really interesting part: There’s another undiscovered Fancy Cat! (Probably another ninja.) Here’s the tweet from CK right after Negato was discovered:
And then a CK guy popped into Discord and told us that the twitter hints (onyx, henna, and wolfgrey) still apply to the undiscovered fancy.
In other words, keep breeding!
UPDATE: The second ninja was discovered! See How to Make Momo-Chan.
To see the recipes for other fancies, check out How to Make Fancy Cats.