Monsters That Are Antagonistic But Not Evil
What's the difference between a species that you play, and one that you don't?
I've been thinking that humans, elves, dwarves, etc might all have cultural differences, but there are also certain things that are universal among humans despite cultural differences. And generally for PC races we think of them as being about the same. So, while you might have differences - some of you fly, others breathe fire - there's a lot of common ground, and so you can roleplay them without too much difficulty, and you can come up with reasons you might all be traveling together and doing things together.
Other ways of being
Let's contrast humans, with, say, cats. Cats are like humans in many ways. They enjoy companionship, but not too much, and they can be picky about who they enjoy spending time with, and take some time to become good friends, like humans. But there are other major differences. Cats like being perched on high places, and like sitting in boxes, and humans usually do not. Cats are generally not fond of water, whereas many humans enjoy swimming, especially in the heat. Cats are attached to their territory and dislike going far from it, whereas humans enjoy traveling so much that given the opportunity they will spend large sums of money to be able to do so. Now, cats don't have human-level intelligence, but if they did, would these things change?
I'm not talking about humans with cat characteristics - I'm talking about literal cats. Humans can be friends with cats, and can have meaningful interactions with cats, but cats are different in a way not conducive to human-style adventuring. You could still have cats as friendly NPCs. Maybe there's a magic shop with extremely limited hours (nap time is important). Maybe you want to learn the secret magic of cats that can see invisible things from the Queen of Cats, but she will help you only if you bring her the Dire Tuna. Why doesn't she get it herself? Well, she doesn't want to get her paws wet, of course.
Cats have their own way of living. They aren't just fluffy humans with tails, and this is something that can make a fantasy world feel more fantastic.
So, the difference between playable and non-playable species becomes whether they are human enough to meaningfully role-play. This is not the same thing as being good or evil, though.
Antagonistic Monsters
Another thing I'm not a big fan of is that some monsters are evil because they have the "evil" tag, which has always felt artificial and video-gamey. Are monsters evil? This is a question that, I think, came later on, when we were all safely in cities behind tall walls. Are foxes evil? Maybe, to the chicken, but they're pretty cute to us, now, that most of us don't have to worry about our chickens being devoured. Are wolves evil?
In the modern world, people are in power, and they may be good or bad, but the question is, how do humans responsibly use that power. We worry about wolves and lions and tigers going extinct. This is a level of power over the natural world unimaginable a thousand years ago, when the howl of a wolf would strike legitimate fear into your heart. This doesn't mean things have to be grimdark - there was joy and beauty in those days, even if it meant that you only travel through the woods in large enough groups.
Monsters aren't about good or evil, they're about the things in the night with the big teeth. They're about remembering that we weren't always at the top of the food chain. But also, if you're having a problem with wolves, you don't have to go kill all the wolves. Maybe you can figure out why they have suddenly started preying on the villagers and solve that instead.
A Take on Bugbears
One of the things with the big teeth is the bugbear.
In D&D, bugbears are traditionally a type of ambush-based goblinoid, who are also very strong. In folklore, they were some sort of frightening spirit that lurked in the woods, possibly cognate with "Bogeyman", according to Wikipedia.
Bugbears are ambush predators. They aren't evil, any more than a tiger is evil. They're large, furry, and mostly like to sleep and eat - not unlike bears in some ways, if bears could talk. They're friendly and nice, if you ever find yourself chatting with one, and coexist peacefully with other bugbears, but rarely form large societies, and mostly keep to themselves. They are not social creatures, by and large, and are perfectly happy on their own, as many obligate carnivores are.
Bugbears are not cannibals - they don't eat other bugbears! They generally won't eat any goblinoids, in fact. Anyone else they can take down is fair game. But they kill for food, and by leaving out food for them and keeping them well fed, they can be perfectly pleasant neighbours. There are villages that leave out offerings for the bugbears and have forgotten why, until they stop leaving out the offerings, and people start going missing. They can even sometimes become fond of certain humans, even defensive of them, in much the same way humans can become fond of a pet chicken or lamb. You could have a friendly, well-fed bugbear NPC in town.
Or, perhaps, a village has made a deal with a local bugbear to have them eat bandits that might pass by. Or maybe eat people they don't like. Or maybe eat travelers and then the villagers come and collect what is valuable, because the bugbear has no use for gold. There's definitely horror potential there as well. After all, bugbears are supposed to be scary.
Bugbear PCs are not impossible, but rare. Keep them well fed.