Likewise, it wouldn't accept “panther” because “tiger” was already there:
> I assume you mean “panther” in the general sense of any big cat.
Why on Earth would it assume mean that, of all things, rather than “black panther”? If it's gonna be pedantic about it, it could've complained about “leopard” and “jaguar” already being there (which they were) instead of complaining about an animal that nobody in their right mind would call a “panther”.
There is no actual "panther" animal though, the word is used for several different animals (leopards, jaguars and pumas at least, I think).
They can all have melanistic coats and are then often called black panthers. But that's not a species.
I believe the poster you're replying to understands that. They're noting that the complaint about panther was curiously because they had already listed tiger, which is practically never called a panther, and not because they already listed leopard, which is a cat that is often called a panther. The statement about meaning "any big cat" I would guess to be a confusion based on the name Pantherinae for the subfamily of Felidae of which all these big cats are part. Though the puma, which as you note is also called a panther, is in the different subfamily, Felinae.
I personally just tend to avoid the word panther, because it very often causes confusion as to which cat you're talking about.