There are a few Easter Eggs within Horizon Zero Dawn referencing other games and media/popular culture
Decima is the engine used to run the game and was created in-house by Guerrilla Games. Its logo is used by both the Eclipse Cultists as their symbol and the Old Ones as the icon for the HADES sub-function within GAIA.
Decima is also being used by Kojima Production for their new IP, Death Stranding. A set of collectibles called Stranded Items, which use the Kojima Production logo as their in-game icon, are objects that were featured in the trailers for Death Stranding.
One of the Ancient Vessels has the Helgast helmet from the Killzone series, a Guerrilla Games franchise.
RIGS: Mechanized Combat League
RIGS is a VR game produced by sister developer Guerrilla Cambridge. Its logo appears on the Denver Stadium in the Vantage Datapoint near Devil's Grief.
In an alcove on the western portion on the map there are rabbits identified by the Focus as Jazz and Jack, in reference to the franchise developed by Epic MegaGames.
Gamers playing the original Horizon Zero Dawn on the PlayStation 4 who managed to get the Platinum trophy (100% game completion) would subsequently get the option from Sony to download a Horizon-themed redesign for the PlayStation 4’s menu screens.
Legend of Zelda
You can find the Triforce symbol at a number of locations within the game.