"These mysterious green-glowing Brassilisks have been showing up in the environment outside the Brass Fortress. Some Clockwork Apostles suspect there’s a rogue Incarnatorium assembling them somewhere out in the Radius."
The Foxbat Brassilisk is a discontinued pet brassilisk for The Elder Scrolls Online. It was available exclusively to Xbox One players with an active Xbox Gold account and can be purchased from the Xbox Store for free until December 31st, 2017. It was also obtainable via code if you took part in the 2018 Summer Game Pass Quest.
The pet was added to promote the Clockwork City DLC, but did not need the DLC to be obtained. The green color scheme of the pet is a reference to the black and green colors of the Xbox One.