Piranha
The Piranha is a light two-seater skimmer (and the best in kamikaze skimmer technology) used by the Tau Empire. It began life as an unarmed low-cost ground transport for commanders and other important personnel, much like the Imperial Guard's Salamander, but during the Taros campaign it was given some extra armor and weapons to serve on the battlefield.
It comes armed with a burst cannon and a pair of gun drones, for mopping up most infantry, though it can swap the burst cannon for a fusion blaster and pack a pair of seeker missiles for hunting vehicles.
Its speed means that it is usually tasked with reconnaissance, though they are also used in rapid-response forces and security teams (loaded for whatever targets they expect to encounter). Though it is not tough enough to take a shot from a tank, it can weather infantry weapons and it is agile enough to harass much heavier vehicles until armor support can arrive.
The model was first made by Forge World for Imperial Armour Volume Three: The Taros Campaign, as an in-universe field test, but it was brought into the fourth edition Tau Empire Codex.
Piranha TX-42
The TX-42 is a slightly heavier variant of the Piranha more suited for hunting vehicles or heavy infantry -- it has no gun drones, but comes with twin-linked fusion blasters by default, with the option to take twin-linked missile pods, plasma rifles, or rail rifles. It also has the same vehicle upgrades available to the standard Piranha variant.