A comment on this question led me to read about the Golden Snidget on Pottermore (emphasis mine):
The Golden Snidget is an extremely rare, protected species of bird. Completely round, with a very long, thin beak and glistening, jewel-like red eyes, the Golden Snidget is an extremely fast flier that can change direction with uncanny speed and skill, owing to the rotational joints of its wings.
The Golden Snidget’s feathers and eyes are so highly prized that it was at one time in danger of being hunted to extinction by wizards. The danger was recognised in time and the species protected, the most notable factor being the substitution of the Golden Snitch for the Snidget in the game of Quidditch. Snidget sanctuaries exist worldwide.
- Fantastic Beasts and Where to Find Them
We see that skilled witches and wizards are easily able to transfigure inanimate things into animals:
Then [McGonagall] changed her desk into a pig and back again. They were all very impressed and couldn't wait to get started, but soon realized they weren't going to be changing the furniture into animals for a long time.
- Harry Potter and the Philosopher's Stone, Chapter 8
So my question is, why would any animal (although I have provided only the Snidget as an example, I'm sure there are be more) be on the verge of extinction if it's fairly easy to perform a transfiguration and create more of them? Are there any rules or regulations in place prohibiting doing this?