Gatebusters
Description: six-foot long centipede-like creatures covered in somewhat offensive-smelling mucus; said mucus is not completely compatible with regular human physiology). Gatebusters seem to feed on the energies that are generated by teleportation or interdimensional travel; the ‘mouths’ filled with razor sharp teeth found on either end of their bodies seems to be merely for defensive purposes. Fortunately, they generally remain passive, only attacking when provoked (which includes keeping them from feeding). Gatebusters are also somewhat impressively armored; military weaponry can shift one, but regular civilian arms will simply just tick the creatures off.
Every civilization that encounters Gatebusters goes through three stages with regard to them: first, there’s horror and disgust. Second, there’s a concerted effort to destroy the horrible things. And third, there’s an increasing willingness to keep them around.
Why the third? Because spatial and dimensional Gates are dangerous. People often wonder how societies fare under conditions where anybody can make a Gate between them and their enemies and throw a bomb through, and the answer is typically ‘not very well.’ Worse, Gates can be made by relatively small groups, or even a particularly obsessive individual. Governments can hope to keep their own forces under some kind of control, and even trust that opposing governments can embrace the local equivalent of Mutually Assured Destruction. But when a crazy entity has access to this tech, things can get complicated.
Under those circumstances, Gatebusters rapidly become a net help; they will rapidly swarm a Gate, when they detect one, and then effectively gnaw it to death. Doesn’t matter how powerful the Gate is, either; the more energy put into the Gate, the more Gatebusters show up. There are actually ways to prevent this from happening, but those ways involve resources that your average crazy entity in its basement-equivalent just doesn’t have — and that are hard for it to acquire. All of this typically ends up instituting an effective ban on Gate technology, without any government having to be the official bad guy in the situation. Governments tend to like that.
One last thing about Gatebusters: they’re not exactly the brightest creatures in the universe or multiverse. In fact, they’re often very good at getting themselves electrocuted to death while eating. If you see their corpses lying around, congratulations: there’s been a Gate in the area lately. It is, as they say, diagnostic.