To give an example..'Forest' biome includes the Owlbear, which has a CR of 3.
Also on that table you have the Gorgon (CR 5) and the Grick Alpha (CR 7).
If I were looking to introduce something that was not a dragon that ate owlbears, I would make them prey for wyverns and trolls: wyverns tend to favour owlbear cubs, and only occasionally attempt to eat an adult; and trolls possess the regeneration ability to survive a fight with an owlbear.
I suspect seeing an owlbear cub being suddenly snatched from above by a wyvern will give your player sufficient sympathy for them.
Through a region was lots of Owlbears would be likely to have less regular bears because they would be competing for similar territory.
There may be monsters out there that are tougher than an Owlbear, but most of them leave Owlbears alone because, well...
The owlbear's reputation for ferocity, aggression, stubbornness, and sheer ill temper makes it one of the most feared predators of the wild.
There is little, if anything, that a hungry owlbear fears.
Even monsters that outmatch an owlbear in size and strength avoid tangling with it, for this creature cares nothing about a foe's superior strength as it attacks without provocation (MM 249) However, owlbears, much like their mundane ursine brothers, can be targeted by humanoids for various purposes.