armor gigas
A giant suit of knight-like full armor wielding a greatsword. Weiss Schnee fought it in the RWBY "White" trailer, adapted for the manga Chapter 3.
In the animation, Weiss eventually learns to summon its arm and greatsword, and later the whole Knight, using her Semblance. The manga reveals that the armor is actually inhabited and controlled by "a possession-type Grimm" similar to the Geist. Like the Grimm, it fades after being defeated.
Volume 4 DVD Commentary reveals its official name to be "Armor Gigas."