There is a spot in Jeju that Koreans call Gue Hae Sun Forest for the actress who took her wedding photos there (with her then actor hubby). This might actually a popular couple photo location, if you have the money to trek out to Jeju from mainland Korea (or of course, you could already live there). You also need to walk around in the forest until you find it.
The scene shot in Arthdal Chronicles is from the opening episodes and involves the horse that Eunsom needs to ride in order to stay with the villagers. The horse is on the ground, looking near dead, and Eunsom is trying to convince the people that you can ride it. They leave him to his nonsense with only Tanya to keep him company.
The location of the spot (and many filming location spots) is believed to be in the forest near Gwaneumsa Temple. Some people have found there way there after the Gue couple had their wedding photos shot there, but they all just say that it is a spot in a forest near the temple. They do not give us any GPS locations (I know, right).
If you plan on making the trek, be sure to take plenty of water, because you might be walking the trail for a minute. Or it could be right away. But you might recognize several spots because many of the Wasan peoples scenes in the opening episodes were shot around this area in Jeju.