I don't see it as obviously wrong. It may not feel wrong to you, and that's fine. But yes, if a loxodon walked in the door I would wonder why it was the only one in existence, where it came from, what's it's story. Because in my campaign it would be odd that there was no historical record of loxodons, no previous encounters. You can only do the "unique one off" races so many times before it would feel artificial to me.
Different strokes for different folks.
The Minotaur was a one off hybrid between a woman and a magic bull in mythos.
People can be polymorphed and cursed, or blessed, by many things.
Naturally, "Far Traveller" background.
A lot of monstrosities are just Wizards screwing around, looking to Mimics.
Harry Potter had potions that changed people which could go wrong.
Gnolls were spontaneously created by the blood of a super-demon, on rather short notice.
Surely more can be imagined. One-off freaks need not feel artificial, that wholly depends on expectation and use. I expect the monsters of the realm have some freakish origins that could set some precedent.
I agree with who you're replying to- the story is the answer to the story's problem. The setting needs to accommodate, but what you list are not necessarily mandatory questions.
IRL, weird things are relegated to History's margins. I bet a world of deep history with magic had weird things happening in the margins that led to XYZ- it would be strange if it didn't: "The only strange day on earth will be the day nothing strange happens at all." I half remembered that quote, so I may have butchered it.
The flexibility of situations like this are why I think the only theme/tone problems come from a DM's approach, or the player's RP.
"Speaking personally" means that I'm speaking about my personal preferences. They are what they are. You're trying to make a point by turning my "is" into an "ought." Cut it out.
I consider calling it "delegation" to be the red flag there. It
may be, but that depends on why the question is asked. If the DM takes your prompt and develops it exquisitely, is it still avoiding their job, or was it just a creative choice?