I'm guessing I must've gotten this on an alt, because I have the Summer Wilderling mount. Is it possible to filter out things like this?

If you want credit for an incomplete quest on one character that another character has completed, use account-wide checkbox for quests, or do the quest again on the desired character. This is NOT a situation where it's impossible do re-do, surprisingly. We don't support blacklisting of arbitrary content in ATT.