The Mechagon Peacekeeper is a new mount added in patch 8.2 of World of Warcraft: Battle for Azeroth, and it is tied to the new Mechagon content in this patch.
The mount drops from a boss called HK-8 Aerial Oppression Unit found within the "Operation: Mechagon" Mythic-only dungeon. The HK-8 Aerial Oppression Unit is the 4th boss, so it's roughly at the halfway point in the dungeon.
Like the other mounts that can drop from Mythic dungeons in World of Warcraft, it seems to have about 1 % drop chance, so you will need a bit of luck to get your hands on this spider mount. Only one player can loot the mount once it drops, but it is tradeable, so you can give it to whomever in the group you want to.