Trenchcoat

A trenchcoat, or trench coat, is a raincoat made of waterproof, heavy-duty cotton gabardine drill or poplin, or leather. It generally has removable insulated lining, ragline sleeves, and the classic versions come from above the knee to meeting at the mid-calves. The garment traditionally is double-breasted with ten buttons, has wide lapels, a storm flap, and pockets that close.

Usage
They were initially used during World War I by Great Britain and France as a replacement to the heavy siege greatcoats in the same war.

Gene, while leading FOX during the San Hieronymo Takeover in 1970, wore a trenchcoat.

The KGB agent Vladimir Aleksandrovich Zadornov wore a trenchcoat when meeting Big Boss, as well as when he was imprisoned in Mother Base up until his death.

A sunglasses-wearing member of Diamond Dogs (implied to be Kazuhira Miller) wore a trenchcoat in 1984.

Big Boss wore a trenchcoat during his time as commander of FOXHOUND and Outer Heaven. He also wore it in 2014.

Multiple members of FOXHOUND wore long trenchcoats with seven button and padded shoulders, including every member of the Sons of Big Boss.

Dead Cell members Fortune and Vamp wore trenchcoats during the Big Shell Incident in 2009. Vamp would also wear it in 2014.

Old Snake wore a trenchoat when meeting Meryl Silverburgh in Eastern Europe in 2014.

Liquid Ocelot wore a black leather trenchcoat with his business suit during his insurrection against the Patriots.

Behind the scenes
Depending on the game/concept art, the color of the FOXHOUND trenchcoat tends to change. As of Metal Gear Solid 4 and the Liquid Snake Play Arts KAI action figure, the official color is tan.

Roy Campbell in the non-canon Metal Gear Solid radio drama wears the FOXHOUND trenchcoat in his artwork.

AbbyShot Clothiers, a company that creates movie and video game-inspired clothing, created two versions of the FOXHOUND trenchcoat: The standard coat, priced at $399 CAD ($385 USD), and a custom sized version is priced at $499 CAD ($482 USD),  both based on it's appearance in The Twin Snakes.