From Team Fortress Wiki
The Horseless Headless Horsemann's Head is a headwear item wearable by all classes. It is the head of the eponymous Horseless Headless Horsemann, complete with tree and eerie glowing inside.
This hat is awarded to players who have earned the Sleepy Holl0wnd achievement, by contributing to the death of the Horseless Headless Horsemann that spawned in Mann Manor. It was available during the Halloween events only.
While wearing this hat, the player is immune to the Horseless Headless Horsemann's "Boo" taunt, but not the fright from the ghost on Harvest Event, Eyaduct or Ghost Fort.
This item can only be worn around Halloween or during a Full Moon; at other times of the year, it does not appear in gameplay, unless the server has Halloween mode enabled. However, it can be equipped at any time and can still be viewed on the loadout screen even when it is not visible during games.
October 27, 2010 Patch (Scream Fortress Update)
- Added the Horseless Headless Horsemann's Head to the game.
October 13, 2011 Patch (Manniversary Update)
- [Undocumented] Fixed Pyro's mask clipping through the hat.
October 27, 2011 Patch (Scream Fortress Very Scary Halloween Special)
- [Undocumented] Horseless Headless Horsemann's Head can now be equipped during the Full Moon.
- There is an unused file that would allow the glow to be team-colored called
pumpkin_hat_blue.vmt. Despite this, there is no texture for a blue glow, nor is the file actually used.
- Like several other Halloween hats, the Horseless Headless Horsemann's Head does not have a randomly selected level; its level is always 31, referencing the date that Halloween falls, October 31st.