Tholian
Tholians were first shown in the TOS episode "The Tholian Web" [1]
In the Enterprise episode "In A Mirror Darkly", Tholians appeared again. Unlike in their TOS appearance, this time their entire body was shown. Tholians are an insectoid species.[2]
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.[3]
Threat assessment
The name of the episode where this alien of the week appeared is taken from the name of their best known weapon. Their ships weave the "Tholian web"--which appears to be some kind of energy net or cage--around the target vessel. It takes so long to deploy, though, that 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 they couldn't leave their position. However, Tholian Webs tend to take much shorter when there are numerous ships present instead of just two.[4]