Blame this.
Cow Ninja
Stats: use the Oxen stats from pg 460 of GURPS Basic Set: Campaigns, but increase DX and IQ to 10, and add Intolerance [Humans], Acting-12, Brawling-14, and Stealth-12.
Actually, by laughing at this you are demonstrating that you fundamentally misunderstand the nature of the ninja. The black pajamas thing is historically inaccurate; ninja were portrayed that way because the bunraku (puppet theater) tradition associated dark clothing with invisibility in Japanese culture. In reality, ninja dressed in a way that was inconspicuous. Or, in the case of cow ninja, in a way that deflected suspicion.