caltrop

1a caltrops also calthrops plural in form but singular or plural in construction :  star thistle 1b :  puncture vine; also :  any of various related herbs (genera Tribulus and Kallstroemia) of the same family (Zygophyllaceae, the caltrop family)

2 :  a device with four metal points so arranged that when any three are on the ground the fourth projects upward as a hazard to the hooves of horses or to pneumatic tires