Tetra
Tetras are a family of small fish found in every tropical fish ever, you know the little ones with a glowy stripe down the side.
40K
The Tetra is a Tau speeder used by Pathfinders to range ahead of their Cadre.
They are primarily used for reconnaissance and infiltration; a pair of Pathfinders will take one (or more frequently, four Pathfinders will take two) to ride out towards the enemy and scout them out. They send information back to their Cadre to help their leader plan the attack. Then, when the main force arrives to engage the enemy directly, the Pathfinders mount back up and join the fight.
Tetras do not have much armor or firepower (in fact, the two Pathfinders themselves pack more heat than the speeder), but they aren't meant for direct combat. What they have is a high-intensity markerlight that lets them call up to 2 seeker missiles each on anything they can see, and a marker beacon that helps them guide Deep Striking units to the battlefield. And that pulse rifles they carry while not terribly good for a vehicle weapons could be fired even after moving full 12", while in comparison pathfinders can only fire either their guns or markerlights.
History
The Tetra model was created by Forge World for Imperial Armour Volume Three: The Taros Campaign, which contained the rules for the Tetra. The rules were rather lackluster at the time, as the markerlight was only single-shot rather than the Heavy 4 "High-Intensity Markerlight" supplied in the update. Following this Tetras immediately jumped into all Tau lists, since they were in all regards superior to Pathfinders. Second edition of Taros campaign nerfed them to more reasonable levels though, once again changing their periscopes to Heavy 2 twin-linked - now they are slightly less efficient than Pathfinders for marker light hits per point cost, while being more mobile and durable, though the general conclusion is that they're still the best marker unit in the game, at least not so horribly OP as before.