After finishing the main story, I looked in the game's case and realised that I had the pre-order version and the war horse. I didn't use the war horse in the main story, so is there any way to get it in the epilogue or do I need to restart?
"In order to get your War Horse, you’ll simply need to progress the main story to a certain point. Once you’ve completed Chapter One and have received the tutorial about stables, you can visit the stable to find your War Horse.
You’ll do this as part of some story missions that teach you all about horses, and you’ll have a choice between a few different horses – some you can buy, some free.
So long as you’ve successfully redeemed your pre-order DLC code that you got for placing a pre-order on Red Dead Redemption 2, you’ll find the War Horse in that list. It’s a trusty grey steed as pictured above, with a white streak down its face. In the menu it’ll be labelled as Ardennes, but you can also double check it by checking its ‘type’ below – it’ll be listed as ‘War’."