If you need to be told who these are, I think you're in the wrong forums.

Anyone who plays a part in the show outside of the mane six. Whether it's the Princesses, Spike or even Shining Armor, they'll be here if they're taken.

Any of the characters that fill the background will go here. You may want to check them here before making assumptions as I'm sure most people envision them in various ways and the person who plays them may not have the same personality you think they should.

Canon characters that were once claimed but are now free again for one reason or another. Anyone is free to claim a character in here as though the character was never claimed. If the character has previously posted (now in the Thread Graveyard), you are free to ignore that or continue it.