Because it was black and, at 5mm (0.2in) was the size of an ant, I thought it was an ant! I started shooting it, then realized it looked different. Not quite an ant. After staring at it for a while, I saw the forelegs and thought, "WHAT! A mantis? That tiny!?"
According to Google, it's a mantis nymph. In this stage of its life, it mimics the ant, possibly as a form of defensive mimicry because dark ants are repulsive to birds that habitually regard the mantis as prey. This nymph will grow up to be a grassy green adult mantis.