The Slayer's Tabard is a Demon Hunter-only tabard that was added at the end of the Legion expansion pack. It's an updated version of the Green Trophy Tabard of the Illidari that could be gotten by questing in Shadowmoon Valley in Outlands.
To get the Slayer's Tabard, you need to complete your Class Hall Campaign up until the point where you get your class mount on a Demon Hunter character. After this is done, the tabard is for sale from Falara Nightsong, the quartermaster NPC in the Demon Hunter Class Hall. It costs 100g, and can only be used for transmog on Demon Hunters.
A high-resolution version of the Purple Trophy Tabard of the Illidari has yet to be added to get game, but it might be implemented at one point in the future.