Arachnid style Evos come in two variations. Magic Arachnid and Noxxious Arachnid. 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.