Altaroth are giant ant-like insects that like to crawl along the ground. They love to eat and forage by areas that have berries or mushrooms. Their stomachs swell when they eat and turn different colors. Killing them when their stomachs are a different color will result in a shiny dropping that could give you items from what they've been eating. If they go to the mushroom area that's one of the few ways to get the Ripened Mushroom, an account item.
If provoked, Altaroth will attempt to bite at you with their massive jaws.
Note that if you want Altaroth to die and leave behind a carveable body, you have to poison them. Poison weapons or shots, or Poison Smoke Bombs are the way to go, with the latter being the most reliable.