Cholla Cactus
This is some nasty stuff. It is covered in barbed needles, which penetrate the skin and steadfastly refuse to let go. Instead, the branches just break off and the piece remains embedded in the poor victim. It is not uncommon to see a free-range cow roaming around the desert with several such branchettes stuck to its head and body.

I had my own run in with these things a bit later on this morning while tracking a Roadrunner.