Your sentence, "Only lions have a fluffy mane; lionesses don’t," is grammatically correct!
Let me explain why:
- "Only lions" means just lions, not any other animals.
- "have a fluffy mane" tells us what lions have.
- The semicolon (;) here is used to connect two related thoughts: lions have a fluffy mane, but lionesses do not.
- "lionesses don’t" means lionesses do not have a fluffy mane.
So, your sentence is clear and correct! Keep it up!