Revenant's are experiments that were created by injecting a corpse with the T-Phobos Virus and the Uroboros Virus. It is alluded that all of the Revenant's were securely sealed away during the time Claire and Moira were exploring the island, but managed to escape by the time Barry arrived.
Revenant's look like severely mutated humans, often appearing with multiple body parts crudely stitched together. Some Revenent's also have crude weaponry embedded onto their limbs, such as knives and serrated blades. They have a weak central core that exposes itself once you have destroyed enough of its body parts.