I need help with Mario Kart Wii, Is this a hack or glitch?

I saw someone using a Mii character using the Cheep Charger, another a Mii character using the Honeycoupe. I've looked on the Mii character and on both and you can't drive those two karts. Earlier today I saw a Mii using another bike that looked like a grandpa with gles that Toadette has. How do they do this? I've unlocked everything and when I check I can't use it on that character. So what's going on?