Insects

No insect can kill a person with its bite, but this does not mean that such a bite will not be unbearable.

An excerpt from the article 17 facts about insects
The most venomous insect is the Pogonomyrmex maricopa ant, which lives in the southwestern United States and Mexico. Twelve bites from this ant can kill a 2 kg rat. They are not fatal to humans, but their bite causes extreme pain lasting up to four hours.