Weighted Jack-o'-Lantern

The Weighted Jack-o'-Lantern item, when used on either another player or yourself, changes your head to a pumpkin (just like the Hallowed Helm effect).

Source
Weighted Jack-o'-Lantern is a common drop from Headless Horseman. The horseman is found in the graveyard wing of Scarlet Monastery during the Hallow's End seasonal event.

It is also found in the Crudely Wrapped Gift (was formerly found in Smashed Pumpkin Loot).

It is also given as a reward for the fire fighting daily quest during Hallow's End.

Achievement
For the Check Your Head Hallow's End World Event achievement, you have to use a Weighted Jack-o'-Lantern on each player race; gnome, blood elf, draenei, dwarf, human, night elf, orc, tauren, troll, and undead, ten in all.