we already know that the creepy crate will be a reward from a low level quest of the same name (coming in patch 4.2), but there's no indication of who or what might offer this quest. it appears to be an end quest of some chain of sorts, but that's all that can be inferred from the quest description.
after checking out the newest quests added (as of 4.2) on wowhead's PTR site, i think i may have found the prerequisite quest chain for the creepy crate. it might start with missing heirlooms, another low level quest.
unfortunately after flying around stormwind city for a while, i haven't found the NPC that offers this quest, so my best guess is that this will either be a seasonal quest chain (and reward) or there are other objectives that must be fulfilled before you can access this portion of the story.
i'm really eager to find out more but i've reached a dead end in my search for answers. the entire chain is especially intriguing for me since it sounds like a plot line straight from a children's novel like nancy drew or some similar detective story. (yes, i was a big fan of nancy drew back in the day... way back in the day lol.)
hopefully we'll see this pet in 4.2 or some time in the near future. maybe a spooky new halloween storyline?