After wondering about the choice of Xamarin for a few minutes, I read this interview with Miguel de Icaza and discover:
"We've used monkey themes for many years," de Icaza said. "Ximian was a play on simian. Mono is Spanish for monkey. Xamarin comes from the tamarin monkey. And we kept the X, though to tell you the truth, I can't remember why we used it in the first place."
That is exactly what I suspected.