Zegion

Zegion (ゼギオン) is an insect Monster that has been found, rescued and subsequently named by Rimuru in the Great Jura Forest.

Appearence
Zegion at first had the appearance of an extremely large Stag Beetle. Later he gained a humanoid form.

Demon Lord Attacks Arc
While strolling through the Great Jura Forest, Rimuru walked into Apito and Zegion, with the two of them heavily injured. At first Zegion didn't want Rimuru coming any closer, but Rimuru managed to convince them to let him help them. The two end up being rescued by Rimuru when he shared some of his Slime flesh and also named them.

Afterwards Zegion started constantly training to become stronger.

The Octagram Soars Brightly Arc
Apito and Zegion were still in the insect form, but they managed to become much stronger than before.