The character on the left is Raijin, what Thundurus is based on. The one on the right is Fujin, which Tornadus is based on. If you look at Raijin, what do you see? Two horns. Looking at Fujin closely will show one horn on its head.
Landorus is based off Inari Okami, which is "represented as male, female, or androgynous." It is "sometimes seen as a collective of three or five individual kami." The horns on Landorus' head most likely represent this.
Hope I helped!