Have you seen a cool emoji on someone's Mastodon post, but you can't find it on your own emoji picker? This may be a custom emoji.
Your instance's admin can add custom emoji too. If you contact them they may be able to add the emoji you want.
There are some useful custom emoji hints for admins here:
There's a tool for viewing instances' custom emoji here:
@feditips Wait, there's custom emojis? *Checks emoji bar* oh my god there is! I'm gonna need to play around with these 😈