Original: The black panther has spots. It is a cousin of a leopard after all.
Corrected (two good choices):
1) The black panther has spots. It is a cousin of the leopard, after all.
2) The black panther has spots because it is a cousin of the leopard.
Why these are better (easy steps):
- Put a comma before the short phrase "after all" when it comes at the end: "..., after all."
- You can say "a leopard" or "the leopard." Saying "the leopard" often means the leopard as a kind of animal (the whole species), which fits here.
- You can also join the ideas with "because" to show the reason: that explains why the panther has spots.
Fun fact: A "black panther" is usually a very dark leopard or jaguar. The spots are still there, but they can be hard to see because the fur is so dark.