What freaky killer animal things should I be watching out for?

I'm in Canada, so the worst freaky killer animal things we have here are, like, grizzly bears. And you can see 'em coming. And they're scary, but they're not like, evil scary. But in the desert, I imagine there's all kinds of killer little things like scorpions and rattlesnakes and vicious man-eating spiders to watch out for.

So tell me, what freakiness lives in the California desert around the polo fields that, upon my knowing of its existence, will encourage me to keep my shoes on?