
In return for pre-ordering Assassin's Creed Shadows, you're treated to the frankly disappointing Thrown to the Dogs quest, which you'll need to complete to claim your pre-order bonus. It's a bit boring, but at least it's incredibly brief.
Before you can bag your goodies, you'll need to complete the prologue and unlock the new hideout system because–spoilers–your reward is a decoration, of sorts.
How to start Thrown to the Dogs in Assassin's Creed Shadows

To claim your pre-order bonuses, you have to complete the Thrown to the Dogs sidequest. This quest can be started by visiting the gravesite on the hill to the west of Tennoji Temple in Izumi Settsu. If you've not been to the area yet, it's to the east of Osaka.
Here, you'll find an injured dog, Tsuki-maru, sitting next to its owner's grave atop the hill. Interact with the dog to begin the quest, and then follow the dog to a nearby farm, find a key under a house, and return to the gravesite. That's it. It takes five to ten minutes to complete this quest. While it's a disappointing quest, I'm glad we're able to give some closure to Tsuki-maru.
Assassin's Creed Shadows pre-order bonus

Once you complete Thrown to the Dogs, you'll earn your exclusive rewards: Tsuki-maru as a pet and 1,000 experience points, which is a hefty chunk of XP in the early game.
If you haven't fiddled with the hideout base-building mechanics quite yet, then I'm sorry to have to deliver the disappointing news that pets in Shadows are effectively just decorations for your home. You can place various animals, including the admittedly adorable Tsuki-maru, back at the hideout. Unfortunately, they don't wander around as you might expect. They'll lifelessly idle in their designated spots, so just try your best to make the area look cosy, alright?