Tholian
Tholans were first shown in the TOS series "The Tholian Web"
In the Enterprise episode "In A Mirror Darkly" Tholians appeared again. Unlike theirTOS appearance, this time their entire body was shown. Tholians are an insectoid species.
Though they often claim (and fight over) territory they do not actually hold, Tholians seem to largely keep to themselves, as their small number of appearances since TOS will support.
Threat assessment
The name of the episode where this alien of the week appeared is taken from the name of their most well known weapon. Their ships weave the "Tholian web"--which appears to be some kind of energy net pr cage--around the target vessel. It takes so long to deploy though it seems utterly useless and can only be used against crippled enemy ships. They nearly ensnared the Enterprise only because Kirk was stranded in yet another alternate reality and couldn't leave their position.