Mascots are famous characters of Club Penguin Legacy who visit the island occasionally, especially during parties/events. They are made more identifiable than other Penguins with their name being in white text opposed to the usual black text color.
If you are able to meet a mascot in-game, you can obtain an exclusive background and a stamp for being in the same room as that mascot.
Note: If a mascot appears on the island as an NPC (Non-Player Character), stamps will not be awarded for being in the same room as them. Stamps are only awarded for being in the same room as the mascots you can interact with. The mascots you can interact with consist of mascots who travel to the different rooms on the island and communicate with the community as they are controlled by the Club Penguin Legacy Team.
To obtain the background from a mascot during a meetup, you will need to collect it from their player card. There will be a box icon in place of where you would normally add a Penguin (the far-left button on their player card) that you can click to collect the background (see picture below).