On the quest "A Meeting with Fate", you are transformed into a crusader to talk give someone a letter, but when you mount up, your mount changes too.
but if you go outside of New Avalon, you won't have the crusader-look-alike-buff, but you'll still have your mount changed, if you're still on the quest and dismount outside of new avalon and remount, you get your old horse back. and also if you mount outside of new avalon and go into new avalon, you will have the DK mount with the Crusader-look-alike-buff.
DA PROOF:
sry if its confusing...
TO SUM IT UP!!!
If you're on the quest and you leave the city, you keep the Brown horse mount.
And if you're on the quest and you're out of the city, you keep your DK mount with the changed body.