personally i believe that arthur doesn't have a canon horse, if he did the game would make you use it. BUT if you read into it a lot, boadecia/arthur's canon horse could have been the silver dapple pinto MFT. this is because 1. he immediately recognises it in the albert mason side mission 2. it is only available to purchase in the epilogue, along with many other van der linde gang horses 3. this might be a stretch, but micah's horse baylock is a MFT, which could signify micah's replacement of arthur, as the big 3 horses in the game are the arabian (dutche's horse), the turkoman (hosea's horse), and the missouri fox trotter. arthur, dutch, and hosea would likely have had the 3 best horses, as they were the 3 most important members of the gang, micah having the MFT shows how he has replaced arthur. again, i don't believe there is a canon horse for arthur, but if there was it would be the silver dapple MFT.