Blade Drone Hive
This would appear when raid losses make it clear to the aliens that Phalanx troops need dedicated robots deployed against them as opposed to the more general purpose bloodspider and bloodmantis.
In this case the robot is actually a stationary AI-equipped "Hive" that would deploy relatively simpleminded five-legged disposable robotic drones the size of a small dog. Heavily armed robotic drones.
Hive Structure: Your average big alien box, about 2 meters on a side, with holes in the side and sprouting spiky antennae.
Blade Drone Structure and Weapons: Five legs, a body containing the power pack and the electronics, a sensor cluster and a plasma blade in the front... and it carries an additional explosive charge.
When Phalanx troops intrude on the raid area the Hive begins dispatching blade drones to deal with what the Hive deems as the most threatening enemy units. When a blade drone is in range it leaps at the target in an attempt to lock on with the plasma blade. If it succeeds it spends that turn killing the target and then it will explode the next turn.
Otherwise it will try again.
And again.
The blade drone's small size and pre-programmed evasive maneuvers will make it a difficult target to hit in the meantime... and there's always the chance that a drone will explode anyway if it is hit hard enough.
The actual embodiment and descriptives can be changed at will, of course, but would this seem likely to provide sufficient game variety for the effort that would be involved?