As Mufasa is in the valley trying to get to Simba through the wildebeasts, Scar is shown pacing above with his shadow cast on the wall up and behind him. Which means the sun would have to be shining on him from below.
Actually, that's not the sun. I'm %99.9 sure it's a fire or some weird animated lighting to make Scar seem a little like the Devil. And they're not wild beasts, they're Hyena's.
The Wildebeests are the antelope-like creatures that are in the gorge, that the hyenas drove down into the valley. I think you're thinking of the later sequence when Simbas (as an adult) confronts Scar. I'm talking about the earlier sequence in broad daylight, no fire, when Mufasa is killed.
They are wild beast! The three hyenas chase them into the gorge. Before the stampede Banzi says "Man I'm so hungry,- I've got to have a wild beast," so they are. I still think you are thinking of Be Prepared where there is fire at the bottom lighting up Scar and there is a group of hyenas marching.
Scar wouldn't look nearly as sinister in that scene if it wasn't lit like that. The lighting was all for effect. It's an animated movie, they can do stuff like that.
Hopefully I'm not being too nit-picky, but the herbivore animals that the hyenas induce to stampede are "wildebeests" not "wild beasts". It's the name of a specific African species.