Their coloring could be different. It's true that most of Morgoth's evil animals (such as wargs) look so similar to normal animals that it's hard to tell the difference. But there are also black swans, Thuringwethil, and Carcharoth.
Perhaps Sauron or Morgoth changes the evil lions to give them a coloration more useful for stealth in forests and plains at night. So they could be jet black. That would then be awkward if they go out during the day after the Sun Rises, though. Dark brown with spots?
The reason I want to avoid the Lost Tales giant cats is because they are, literally, huge house cats. Which have proportions that work at a small size but would look silly at giant size and also not make them graceful. And because housecats are cute.
Perhaps Sauron or Morgoth changes the evil lions to give them a coloration more useful for stealth in forests and plains at night. So they could be jet black. That would then be awkward if they go out during the day after the Sun Rises, though. Dark brown with spots?
The reason I want to avoid the Lost Tales giant cats is because they are, literally, huge house cats. Which have proportions that work at a small size but would look silly at giant size and also not make them graceful. And because housecats are cute.