Animals that can sting:
Ants, Bees, Wasps, Scorpions, and Jellyfish

If we define a sting as a venom injected *at will* for defense or hunting this ability has evolved multiple times. Cnidarians were first with their stinging cells called nematocysts. I believe the hymenopterans were next?

(are there any other insects that can sting that aren't hymenopterans?)

Should the bite of snakes or the bite of spiders count as a "sting?"