Obtaining the Amalgam of Rage
To obtain the Amalgam of Rage, you must purchase at least the standard edition of Diablo IV. It is currently listed as a reward for pre-purchasing Diablo IV but it is unclear whether the Amalgam of Rage will be removed once the game launches.
However typically mounts of this nature are either kept as a purchase reward, for example the Meat Wagon from purchasing Warcraft 3 Reforged or they are brought back in bundles or as individual purchases.
Once you have purchased Diablo IV, the mount will be automatically added to your battle.net account and you should find it shortly as a gift in your mount log within the game.