Cane Toads
Originally introduced to control pests, the cane toad has become a pest of its own. Native to Central America, the toads were brought to Australia in 1935 in an attempt to control the cane beetle population in sugar plantations. Ultimately there was no evidence they killed a single beetle.
Instead, the toads took over. Cane toads have few natural enemies outside of Central America, and when other animals try to eat them, sacs that run down their sides secrete a poison that kills predators in minutes. The toads' voracious appetite depletes resources for other native animals, and they're even known to steal pet food from bowls left outside houses. Weighing up to 3lb. (1.3 kg) and measuring up to 6 in. (15 cm) long, these toads are serious threats to ecosystems in not only Australia, where they number in the hundreds of millions, but also in Florida and parts of Texas.