Several people have reported that the Sinister Squashling can now be rarely looted from a [Treat Bag], obtained by trick or treating with the innkeepers. It can also be rarely found in the [Crudely Wrapped Gift] rewarded for the daily quest involving putting out the Headless Horseman's fires. This is welcome news for pet collectors who have not had luck getting this pet from the Headless Horseman, despite the apparent increase in drop rate (with a range of 3-14% listed in Blizzard's armory).
It appears that the Hallowed Helm can also be obtained in the same manner as the squashling. A player that is fortunate enough to collect both items will complete the Sinister Calling achievement. If you are really in the hallowed spirit, you can try to complete this and all of the other achievements required for the achievement Hallowed Be Thy Name, which gives the title The Hallowed. Happy haunting!