Invite Friends to get Moneko was a feature in The Battle Cats that allowed you to invite friends and unlock Moneko and Bean Cats. It was removed, probably due to the number of friend code spamming on PONOS' media and other Battle Cats-related sites.
This feature was about inputting friend codes of other players to receive in-game rewards, including Moneko. The feature was available since the very first version of Battle Cats. In version 2.3, a second unit, Bean Cats, was added. Later, PONOS decided to remove this feature and make these cats drop rewards from Empire of Cats Chapter 1.
After clearing Japan in Empire of Cats Chapter 1, the player would receive 2 codes and a new button would be added to the title screen, labelled "Invite friends to get Moneko]]. It allowed the player to input codes such as Serial Codes.
For the Moneko code, after 10 players have input it, Moneko would be unlocked.
For the Bean Cats code, after 3 players have input it, Bean Cats would be unlocked.
Serial Codes: Since the button on the title screen was replaced with the mailbox, codes received from Merchandise are unusable.