A non-human species who breathe boron gas, whose ship collides with Enterprise in an asteroid field, leading Tucker and Reed to believe their own ship lost in Shuttlepod One.
The Tesnians are mentioned in the Enterprise episode Shuttlepod One. In 2151 a Tesnian vessel loses control near an asteroid field while attempting to dock with Enterprise, strikes the launch bay doors and crashes on one of the larger asteroids. Its crew are rescued, and Archer takes them back to their home world.
The incident sets up the main story. Tucker and Reed, away in a shuttlepod with damaged sensors, find wreckage on the asteroid that includes pieces of Enterprise's hull and conclude that the ship has been destroyed.
The Tesnians themselves are not shown. All that is said of them is that they breathe boron gas, which made housing the survivors aboard a human ship awkward, and that Enterprise had to leave the area to deliver them, which is why the ship was missing from the rendezvous point when the shuttlepod returned early.