he dismounted in the last battle though
Yes, but the warriors from Willendorf literally just left him alone last we saw. He should have already gotten back in his horse, unless those few panels are happening right before...
But wasn't that horse which had laid down on the few previous panels the same horse? So it would have been more than just a few minutes...which, again, feels weird. Since by that point Polidoro should at least be close to his horse again, being his method of transportation this close to enemy territory.
I don't know, those last few panels muddle the timeline of events for me, enough that I don't know if it makes sense or not that the horse would have laid down like that in the time it took Polidoro to beat Caroline and what it would take to go back to it while carrying her corpse.