Horizon: Dawn of Zero launched a DLC, Ice Dust Snow Field, the year after its release, and the latest news suggests that the week-long sequel, Horizon: Desperate West, may have taken a similar strategy.
The VSC Ratings Board (the agency responsible for rating games in Europe) recently posted a video on Twitter about how horizon: Desperate West's PEGI16 age rating is rated, and the video mentioned that Horizon: Desperate West includes an "in-game store" for purchasing "subsequent DLC extensions." As anyone who has played the game will probably know that Horizons: Desperate West doesn't include an in-game store, it's almost a sign that DLC exists. I don't know if it was the staff who said it was a slip of the tongue.
This DLC has not yet been officially announced, the authenticity remains to be seen, what the specific content of the DLC is not yet known, the official is now busy fixing the various bugs in "Horizon: Desperate In the West", and should not soon disclose the follow-up content.