Now, the having things like the horse as an Uber-rare drop (and thus a Feat of Strength in the achivements) is fair enoughand adds to their value, but the pumpkin hat and pet are required for the "Sinister Calling" achievement (which in turn is required to complete the "Hallowed be Thy Name" achievement (which in turn is a requirement for the "What a Long Strange Trip its been" meta-achievement).
I'm talking 40-odd summons, and only one of the items dropped (twice as it happens) . Add in the competition factor (assuming everyone rolls, a 20% chance of winning the item) and the odds of being able to achieve "Hallowed be Thy Name" are rather slim.
Most achievements are completable through time and effort. This one is time and luck - not a good combination - and there're plenty of stories on the forums of dishonest dealings by people wanting to get the items (pretending to have summons, for example).
Blizzard have already said they're going to pull the other "pure luck" achievement from Hallowed be thy Name ("A mask for all Occasions" which requires you to get all 20 Hallows End paper masks .. and I've managed a grand total of zero so far this festival ...) so here's hoping that they take out Sinister Calling too.
Addendum: It appears its "working as intended"
Addendum 2: There are reports the items may be attainable via trick-or-treating