Fenar pulled his horse to stop at his brother’s signal. Torkaz stopped, too, not quite as quickly.

“There, by that rock. Look close,” Vatar said.

Fenar looked where his older brother pointed. First he made out the darker mane of the male lion, then its huge body. Three females lay nearby, watching the three boys with mild interest.

“What do we do?” Fenar asked, proud that he kept his voice steady despite the appraising gaze of the lions.

“Nothing. They’ve eaten recently. They only want to be left alone. We just ride on.”

“How can you know that?”

I think you could trim this a bit and work in your hook. Is it vitally important for us to know that Torkaz doesn't stop as quickly as Fenar? For a moment I stopped to wonder if Torkaz was the brother who made the signal to stop. I think you could clarify this and cut it down to something like, "The two boys pulled their horses to a stop at Vatar's signal." or something like that to save yourself a few words and simplify the opening. You could work in which two are brothers at a more natural point later on.

First he made out the darker mane of the male lion, then its huge body. Three females lay nearby, watching the three boys with mild interest. This reads a little strangely to me--he can barely make out the mane and then suddenly he's seeing the body and then three females. I think you might be better served by giving us a setting while describing the lions. All I know at the minute is there is a rock with some lions next to it. Where are we? I think you could better describe the lions and how difficult it is to see them if you use the surroundings, such as the shadows dappling through the trees or something like that.

Aside from that, I can't help wondering why this would be a good place to start the story, whether four idle lions doing nothing are worthy of the opening to a short story. If the point of the opening is to show Vatar's connection to lions, I think you could do it in a much more active way.

I'll read the whole thing if you like--I've (amazingly) got a couple of free evenings coming up to catch up on some crits.

The effort and slow resolution of making out the lions would depend on distance and background contrast. Is the distance between the boys and lions important later in the story? If so, it might be worth mention while they are trying to see.

I think you need more setting to this to help ground the reader. When Fenar looks for the lion describe the scenary a little before he can make out the lion on the rock. What color is the rock? Is there long grass? Are they on a savannah? I also want to know how far away the lions are from the boys. If the lions are really close to them that adds more tension.