Enter the following commands in chat while you are selecting your character.

Goat:
/BAAAAHHH!!!
-
Gangbeast:
/gangbeast
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Octodad:
/octodad
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Payday Characters:
/cloaker
/dallas
/hoxton
/wolf
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Potato:
/sallad
-
Qwop:
/qwop

Barrel Duck: -> (for Pewdiepie)
Comes only in a special build of the game that the devs choose to give to. The code is "/pewduckpie" but it will only work on that special build.

MissingNo:
The Missingno Pokemon, which stands for Missing Number, is a Pokemon that has data that only partially exists resulting in a glitched character sprite. This character can be accessed by replacing game files, but it will crash your game. Whether the reason why it crashes is because of it being not fully implemented or as a joke by the developers is uncertain.

Please, Like and Favorite. I will be updating this guide when needed.