Thundercats depicted a race of catlike humanoids, not earthfolk. But we sense that Panthro is black—even though he can't be in the continuity of the show—because of how the character is coded.
Monique Jones explains at Color.
Let's start with the coloring. Most of the other Thundercats are in shades of orange or (literal) white, have spots, stripes or whathaveyou. Lion-O is the only Thundercat that's an earthly flesh tone color. It's also interesting that Lion-O's skin tone is the only tone that corresponds with an earth-bound "white" character. Meanwile, Panthro is a blue-ish gray tone; his coloring already "others" him from everyone else, while Lion-O is colored in a way to signify to the audience that we, as humans, are supposed to identify with him. In other words, Lion-O's coloring is another way the media uses whiteness as the default for characters.
Monique doesn't mention Panthro's fabulous theme tune, reminiscent of early disco from before when the funk evaporated off.