Tetras are a family of small fish found in every tropical fish tank ever (you know, the little ones with a glowy stripe down the side).
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.
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.
The Tetra is in a weird place now. On one hand, if it scores the hit with a markerlight, it basically counts as three, has T5, great movement, and Pulse Rifles. On the other hand, they only have a 4+ save and T5, and only have one shot with the markerlight.
Niftily enough, they have a deepstrike equivalent move which allows them to be set up 9" away from enemy models, but this messes with your BS with the Markerlight. In general if you want a vehicle with Markerlights, the Skyray is a better option.
|Forces of the Tau|
|Command:||Cadre Fireblade - Ethereal - Ethereal Guard - Tau Commander|
|Troops:||Drone Squadron - Fire Warrior Team - Pathfinder Team|
|Auxiliaries:|| Great Knarloc - Gue'vesa - Kroot Carnivore Squad - Kroot Hound |
Krootox - Lesser Knarloc - Nicassar - Vespid Stingwing
|Battlesuits:|| Battlesuits (XV02 Pilot Battlesuit - XV15 Stealthsuit - XV22 Command Suit |
XV25 Stealthsuit - XV46 Vanguard Void Suit - XV-8 Crisis Battlesuit
XV88 Broadside Battlesuit - XV9 Hazard Battlesuit - XV95 Ghostkeel Battlesuit
XV104 Riptide Battlesuit - XV107 R'varna Battlesuit - XV109 Y'Vahra Battlesuit
KV128 Stormsurge Ballistic Suit - KX139 Ta'Unar Supremacy Armour)
|Vehicles:||Devilfish - Hammerhead - Piranha - Sky Ray - Tetra|
|Flyers:||Barracuda - Manta - Orca - Razorshark - Remora - Sun Shark - Tiger Shark|