Translation:What is the most famous animal in China?
If "What Chinese animal is the most famous?" is accepted, it seems reasonable to allow students to drop the "the" and accept "What Chinese animal is most famous?"
One needs THE for most famous, or best, or hardest. I suspect in colloquial English, the THE might be dropped.
Actually this is a matter of how we conceive of the adjective, in this case "famous".
With "the", the adjective is nominalized, as the implied noun (in this case "animal") has been elided from the end of the sentence, and the adjective remains to perform its function.
Leaving out "the", on the other hand, is valid where we treat the adjective as simply predicative, which is a normal way to use adjectives.
It's undoubtedly true to say that the nominalized version is much more common for this kind of superlative expression, but both versions are grammatically correct.
(However, if we're going to call one more colloquial, I'd suggest it should be the nominalized version.)
Should accept "animals" as well as "animal". (With "are" instead of "is" of course.)
Two animals can be equally famous, and "the most famous animals" is just as grammatically valid as "the most famous animal", so "the" doesn't necessitate the singular.
"Which animals are the most famous" could also be a question about the two or three most famous animals among a larger group of relatively famous animals, even if, within that top tier, one were still more famous than the other(s).
This entire lesson -- Culture 1 -- needs to go back to the drawing board.