I’m a fan of the animal burger at In-N-Out, but that is the exception that proves the rule.
What really matters is getting the main menu squared away. Make sure each item is a blockbuster (or get it off the menu). Do it again tomorrow. And the day after that.
Once you’ve got that set, then you can play with these one-off items, or if you want to be cute, go ahead and add something on to the secret menu that is actually a main menu item, just unwritten.
But let’s be honest, how many of us actually got the main menu down?