These are the flying mounts you can acquire the Necorlord Covenant.
Battlefield Swarmer Harness: You can receive this flying after completing the One the Offensive achievement for completing covenant assaults in the Maw activities. It will cost 1,000 Stygia.
Lord of the Corpseflies: This flying mount will drop from a Fleshwing in Korthia. The Fleshwings spawn during the Corpse Heap Necro-lord event.
Maldraxxian Corpsefly Harness: You can purchase this mount once you have reached renown level 45 for the Necrolord Covenant.
Regal Corpsefly Harness: You can speak with Su Zettai in the Seat of the Primus to unlock this mount. You must reach renown level 70 with the Necrolord Covenant, and you have to pay 7,500 Reservoir Anima.