User blog comment:FairyRaven/Fairy Tail Chapter 327: My Part Too, Review and Commentary/@comment-7841492-20130412131019/@comment-7418318-20130412132524

Acnologia is not Zerefs pet. Zeref seemed pretty pissed about Hades "summoning" that dragon and quite a bit unsettled about being so near to it.

As for how f-Lucy forgot, let's consider what she's gone through. She lost her right arm and was knocked out and when she awoke again some indeterminate time later, all other peopel who were present (which includes her close friends Wendy, Happy and Charla) were dead and the only thing visible as far as the eye coud see was one massive scene of destruction. For all she knew, everyone of her friends and comrades was dead. Considering her reaction to seeing Natsu during the match with Sting and Rogue, she probably found his corpse before going back through time. It's hardly surprising that such a massive trauma and shock would cause a temporary amnesia.