Let's look at the sentence: Only lions have a fluffy mane; lionesses don’t.
This sentence is telling us that only lions (the boy lions) have the soft, fluffy hair called a mane. The girl lions, called lionesses, don’t have this fluffy mane.
Grammatically, the sentence is correct! It has two parts joined by a semicolon (;), which is used here to connect two related ideas.
So, the sentence means: Boy lions have the fluffy mane, but girl lions do not.