These are small floating, mechanical gunships dubbed "hovernets" by those who survived the first encounter with these things. They are aptly named - they hover above the ground through unknown means, and like hornets then can be extremely dangerous. They are armed with a light tachyon cannon. They can fly up and down without any restrictions, and they move very fast. It appears these machines are a sort of security system, deployed in case a landed or crashed UFO is attacked.
Hovernets are remote-controlled, and only appear in landing/crash missions (for UFO's that have them on board) and alien base missions. Hovernets are mechanical and should be researched in a equipment lab rather than a biolab. Shooting them will destroy most of their internal systems. To capture one "alive", the human player must use shock damage weapons to overload the Hovernet's systems.