The highwayman mask is a black mask similar to the ones highwaymen wear. Some players argue that it is very different, but when shown on a player while in game, they look almost identical to the ones worn by the NPC highwaymen. However, the only highwayman to be featured in chat is Rick Turpentine, who looks like the player in chat.
Players can obtain these masks from Easy Treasure Trails. The mask can be stored in the Treasure chest, which can be built in the Costume room of a player-owned house.
These masks, like many other accessories, do not give any combat bonuses. Players who have completed the Cabin Fever quest can take the mask and a Black cavalier to Patchy on Mos Le'Harmless, and have them sown together to create a Cavalier and mask for 500 coins. The highwayman mask can be worn along with a plain black Cape, brown Fremennik shirt, and Steel sword to resemble the highwayman himself.
A player wearing a highwayman mask
For an exhaustive list of all known sources for this item, see here