The Rat Mask is a promotional item launched during the Lunar New Year event in January of 2020.
Alongside the Dragon Door Knocker and Chinese Lantern, this was a limited-time purchase available from the in-game RUST store. It has since been discontinued for purchase but can be traded for via the Steam Marketplace.
From a statistically functional perspective, this is a basic reskin of the Wolf Headdress complete with the same cost and defensive capacity. Unlike said headdress, it crafts considerably faster when not under the effect of any tier workbench. When paired with the Bandana Mask, it provides projectile protection equal to the Coffee Can Helmet.
The only way to acquire this item in-game aside from administrative F1 Console spawning is to own the Steam item and craft it.
The rat mask’s unique appearance makes the wearer resemble a person wearing a huge rat face on their head, striking fear into the hearts of people that hate rodents everywhere.