Ya' know ever since I saw Smartypants under Appleblooms bed I had a theory. What if back when Twilight did the spell Big mac was actually immune to it. and the only reason he fought everyone for it was because of Appleblooms want for it. Upon being released from the spell he then proceeds to take it to repair it for Applebloom. fast forward to when the CMC are journalists he gets upset because she takes a picture of him "playing" with the doll when he was actually preparing it for a gift for her. He then gives it to her as a gift seeing that she wanted it so badly before. Possibly during the off season between the 3 and 4 seasons. Season 4 comes around and Applebloom has long ago misplaced it and it remained missing until Applejack found it under the bed.
Low, seeing as she tethered him to at least one house and it didn't slow him down, if he gets one solid hit in she's done, and I doubt they'll find enough of her to bury. If, on the other hand she can avoid him and perhaps nail his sweet-spot(>.<) she might have a chance.