The description of a "comeback" suggests this is not a brand new version of the Brutosaur, though the string doesn't specify exactly which of the two previous Brutosaur mounts are returning:
Less likely is the the original Reins of the Mighty Caravan Brutosaur. Available during Battle for Azeroth with a hefty 5,000,000 gold price tag that put it firmly out of reach for most players, the mount was removed at the launch of Shadowlands in a widely publicized "extinction event." Though added to the Black Market Auction House, it remains exceedingly rare.
More likely is the recent Reins of the Trader's Gilded Brutosaur, which sold for $90 during WoW's 20th Anniversary, making it WoW's most expensive individual in-game shop item to date. Though publicly derided due to the high cost and near-exclusive services provided, the mount proved to be obscenely popular, speculated to have generated over $15 million in sales over the three months it was available.
While either mount could "comeback," it would seem very unlikely that the original Brutosaur would transfer from an in-game gold purchase to a store item, making it seem almost certain to be the mailbox-toting Trader's version, probably with the same $90 price tag. As for when it may return to the store, its a good bet that it will coincide with the upcoming WoW's 21st Anniversary event later this month.
