tenzin rolled a 18 investagation looking for more exotic mounts
With Tenzin’s keen eye for detail, he notices a small, discreet stable towards the back of the market, guarded by a heavy wooden gate. As he approaches, an elderly woman, who appears to be the stable’s owner, greets him. “Ah, you have a good eye, young one,” she says with a smile. “You must be looking for something special. I do have some exotic mounts for those with discerning tastes and a willingness to pay.”
Honestly, this seems like a reasonable outcome. ChatGPT says they might be hard to find, character rolls an 18, and he finds them. What do you think should have been the response? "No, you don't get to roll to search for them"? The DM set up a potential hidden item, the player rolled high enough to find it, voila.
Compare to when he's trying to steal one of them. He rolls high enough stealth, but fails animal handling. To avoid being caught, he has another roll that allows him to cause chaos by opening up a stall and allowing him to escape. That all seems like a pretty well fleshed out scenario.