The focus of this week's Wandering Monsters is less monsters and more questions that can all be answered with the following statement: it depends entirely on the Dungeon Master and her setting.
Is it okay to kill orc babies? Do orcs even have babies? Both depend on the setting. In a lot of settings they seem to be used as a convenient monster that players can kill and rob without having to think about it too hard. Kind of like zombies, just stronger, faster, and technically smarter. Simple and straightforward, and there is nothing necessarily wrong with that because...
...you do not have to explain what they are or where they come from.
Orcs can just be destructive humanoids that randomly surge from beyond the mountains to plague "civilized" lands. Do they have babies? Who knows. Maybe they are corrupted elves, grow from Gruumsh's lost eye, were cursed by a fiend (or angel), or worship a god of destruction. The point is if your players are never going to go over those mountains and try to put a stop to them once and for all, then why bother thinking about it too hard?
Well for one there are plenty of monsters that can already fill the role of guiltlessly-slayable humanoids, and do it better: most undead, fiends, and constructs come to mind. Lumping in orcs—and goblins, drow, gnolls, kobolds, etc—with them seems pretty boring and lazy. One of the many, many reasons I enjoy Eberron is because it made monsters more interesting, giving them a culture and involving them in the lives of the "civilized" races beyond ugly Medium-size humanoid bag o' XP and cash.
My other gripe concerns D&D's laughable alignment system: how does an "often Chaotic Evil" society even operate? There are plenty of parents in our society ill-suited for rearing children, but I cannot imagine that they would regularly perform worse than your average orc. If I wanted to throw orc babies into the mix and I had to use alignment (which I thankfully do not) I would make them Neutral/Unaligned; there are more creative ways to bring them into conflict than "because Evil!".
I was unaware that in any Dungeons & Dragons source ever that someone tried to pitch the idea that dragons evolved from dinosaurs (which I prefer just being called behemoths, or something else). While I think there is a very good reason that both 3rd nor 4th Edition omitted that even as a potential theory (it sounds really stupid), does it really matter if dragons are reptiles, mammals, or something in between? Is it going to change anything? Who gains from either answer?
As with orcs, perhaps even more so because they are dragons, you do not have to determine where they come from. Their origins—and motivations, goals, capabilities, etc—could be a complete mystery, especially since it is very unlikely that a campaign is going to explore them: dragons just are, always have been, and will always be. Of course if I make the effort to establish an origin, you can bet that I am going to err on the fantastic side of things, having them born from gods, if not created around the same time that gods came into existence.
I really do not get the hangup on dragonborn. I have no problem with "dragonboobs" because like dragons—and really D&D things in general—they do not have to abide by real-world classifications. Again, does it really matter? Does anyone really care? My only issue with them is that they "officially" all look the same. In my campaigns Bahamut and Tiamat created their own race, which have scale colors, crests, horns, tails, etc similar to those of true dragons.
When it comes to combined monsters I am kind of on the fence. I have nothing against wizards doing weird experiments with weird results, I just find it hard to believe that a wizard created enough owlbears to make a self-sustaining species, though I would love to have a kind of chimeric toolbox to make it easy for Dungeon Masters to build one-off monsters to throw at their players.
Most of the time I see combined monsters as not combined at all, but just as "natural" as the animals that we expect to see in real life: griffons, hippogriffs, sealions, displacer beasts, owlbears, etc. If you had to create an origin, I would run with "naturally occuring", nature spirit, god, or extraplanar transient before checking out the "wizard did it" angle. Maybe a magical event merged a bunch of stuff, but that might be stretching it.
Finally, races. The more the merrier. Make as many as you want, touch on their personality, appearance, and culture, but do not try to tell me which ones are common or unusual. In an upcoming setting I am working on dragonborn and devas are a pretty big deal, while halflings and dwarves are right out. Group them up thematically or whatever, but ditch the rarity system/unusual label; it serves absolutely no purpose except to potentially throttle the creativity of new DMs.