Revenant cyclopes are the undead versions of cyclopes slain in the God Wars. They are more dangerous than their combat level suggests, more than a player with the same combat level.
Like all other revenants, they can restore lost health, can cure themselves of poison and react quickly to a player's protection prayers/curses.
Historically, these monsters, along with all other revenants, could freeze you from movement and block you from teleporting, but after the update on 1 February 2011 that relocated them to the Forinthry Dungeon, they no longer can do either of these.
The Forinthry Dungeon is a PVP-enabled area, meaning it is entirely possible for other players to attack you whilst fighting the revenants in the dungeon. Do not bring equipment that you are not willing to lose.
Revenant cyclops were given a different set of sounds for being attacked and death to a player with the release of the Evolution of Combat. The only other revenants that were affected by this were the Revenant icefiend and Revenant pyrefiend.