An Erprobungskommando (EKdo) ("Testing-command") was a variety of Luftwaffe special-purpose unit tasked with the testing of new aircraft and weaponry under operational conditions.
[1] The similarly-prefixed Erprobungsgruppe 210 was meant to have service-tested the Messerschmitt Me 210 twin-engined "destroyer" and light bomber, but that design's early aerodynamic problems caused the unit's transformation and expansion into SKG 210, a dedicated "fast bomber" unit using Bf 110s instead.
Erprobungskommando 16 was formed March 1943 in Peenemünde-West, as a test unit for the new Messerschmitt Me 163 rocket fighter, and later based at the Luftwaffe airfield in Bad Zwischenahn for a considerable period of time.
Erprobungskommando 154 was formed in November 1943 in Langenhagen, to test the new Focke-Wulf Ta 154 night fighter.
[2] Zirkus Rosarius (also known as the Wanderzirkus Rosarius) was an Erprobungskommando-style special test unit of the Luftwaffe, specifically of the Luftwaffe High Command, tasked with testing captured British and American aircraft, all of which were repainted in German markings.