You must live in warm climate where oleanders grow. Florida? That's the oleander caterpillar. No, they do not sting, if that is what you mean by poisonous. I suspect, however, that they pick up the toxins from the oleander, making them unsuitable for many predators, including birds, to eat.

These caterpillars are never found in ones or twos. They travel in big packs, so remove them as soon as you find them.

Truth be told I have never heard of a poisonous caterpillar.

There are many species of poisonous caterpillars. Some are much more of a problem than others. Their dangerous aspect lies within spines located along their bodies, usually hidden by tufts of hair. Some species are simply very irritating when touched; others can send sensitive people to the hospital.

First we have to differentiate between "poisonous" and "venomous". For example a poison arrow frog is poisonous while a rattlesnake is venomous. Poisonous can be stretched a bit to include "unpalatable". A great example in the caterpillar world is the monarch. It picks up toxins from the milkweed which is its sole diet. There is also the defense mechanism of spines and mimics. This caterpillar appears to use multiple defense mechanisms. One is the black hairs that mimic spines and tell a predator that it would be an unpleasant meal. The color and stark contrast of black on orange also says "I don't taste good". Such is the marvel of nature.