I recently tried to evolve my halloween charizard mount into its mega form (using the NPC in celadon market for 400,000 poke dollars) and the mount is nowhere to be found in my bag.
What i have instead is 2971 unknown itemname thing. Mega halloween form also can not be previewed in the store.
Figured its temporary before the chari mount rework kicks in, but would like a confirmation on this.