Arachnid style Evos come in two variations. Magic Arachnid and Noxxious Arachnids. Evo eggs can be obtained from defeating Ancient Arachnids in the far reaches of the Spider Cave. It is a rare drop so plan on spending some time in the cave if you really want one. These Evos are able to be mated so you may be able to find players selling the eggs if you do not want to forgo the hunting. Once hatched you will need to start the long process of training your Evo. It is often best to train your Evo on weak creatures like skeletons through the first two stages in order to obtain the best possible stats.
Noxxious Arachnid Evos come with the ability to lay some really heavy poison on its prey but this added ability comes with a cost. The Noxxious variation is not as powerful in terms of hit points or the physical damage it can deal out when compared to it's Magic cousin. It also takes more damage with a reduced Armour rating (example stats posted below). It's a trade off but when used right the Noxxious Arachnid Evo can be one of the most effective damage dealers in game.
As with all Evos, Arachnids come in many different color variations.