Neopet's chat boards have been revamped into some thing called NeoBoards. There's an option to add avatars next to your name to personalize and distinguish yourself. But because millions of other users have the same avatar, it's not really unique. That is why Neopets made secret avatars, only usable when you unlock it! Here is a list of the secret avatars and how to get them :)
 

Illusen's Glade Avatar Complete Illusen's Glade Quest 20	Adam Avatar Visit the 'About Us' page with the same amount of np Adam has
Jhudora's Cloud Avatar Complete Jhudora's Cloud Quest 20	Queen Fyora Avatar Buy an item from the Hidden Tower
Math's Nightmare Avatar Receive a high score (trophy) in Math's Nightmare	Wheel of Excitement Avatar Land on 10,000np at the WoE
Zurroball Avatar Receive a high score (trophy) in Zurroball	Dr. Sloth Avatar Random event
Sidney Avatar Unreleased	Coltzan Avatar Have him raise the HP of your neopet
NeoPets Version 2 Avatar Unknown	Extreme Potato Avatar Receive a high score (trophy) in Extreme Potato Counter
Wand of Dark Faerie Avatar Equip a Wand of the Dark Faerie to your neopet	Snowbunny Avatar Unreleased
Doglefox Mud Avatar Unreleased	Doglefox Sleep Avatar Unreleased
Harris Sleep Avatar Equip between 24:00:00 pm and 1:00:00 am NST	Kadoatie Sulk Avatar Equip a Kaodotie petpet to your neopet and refresh the pet reference page
Pet Rock Avatar Unreleased	Meuka Snot Avatar Unreleased
Zurroball Avatar Unreleased	Brain Tree Bad Day Avatar Complete Defenders of Neopia mission 5
Meowclops Avatar Equip a Moewclops to your neopet  and refresh the pet reference page	Rainbow Faerie Avatar Complete the Rainbow Faerie Quest
Soup Faerie Kind Avatar Continuously feed your pet at the Soup Kitchen	Chia Clown Avatar Unreleased
Kauvara Avatar Unreleased	Jeren Hero Avatar Unreleased
Kasuki Avatar Unreleased	Ghost Lupe Spooky Avatar Unreleased
Garoo Cease Avatar Unreleased	Earth Element Smash Avatar Unreleased
Dung Avatar Unreleased	Capara Avatar Beat Capara in Cheat
Usukicon Shop Keeper Avatar Visit Usukiland Advert	Usukicon Usuls Avatar Visit Usukiland Advert
Faerie Grundo Avatar Visit a Faerie Grundo's lookup	Aisha Pirate Avatar [Click Here]
Shoyru Pirate Avatar [Click Here]	Krawk Pirate Avatar [Click Here]
Scorchio Pirate Avatar [Click Here]	Better Than You Avatar Complete a Better Than You Competition
Grundo Snow Throw Avatar Receive a high score in Grundo Snow Throw	Grey Faerie - Fallen Avatar [Click Here]
Anglepuss - Angel Avatar [Click Here]	Maraquan Krawk Avatar Visit a Maraquan Krawk's lookup
Anglepuss Avatar Equip an Anglepuss avatar to your neopet and refresh the pet refence page	Defenders of Neopia - Lupe Avatar [Click Here]
Defenders of Neopia - Aisha Avatar [Click Here]	Dr. Death Avatar [Click Here]
Techo Master Avatar [Click Here]	Chocolate Avatar Purchase a chocolate from the Chocolate Factory with a rarity of 90 or higher
Escape From Meridell Castle Avatar Receive a high score (trophy) in Escape From Meridell Castle	Space Faerie Avatar Defeat the Space Faerie in the battledome
Mad About Orange Avatar Hold 8 items with the word 'orange' in your inventory and refresh your inventory... a lot!	Codestones Avatar Have the complete set of 10 codestones in your inventory
Pant Devil Taunt Avatar Hold a Hidden Tower artifact in your inventory (very risky!) or trades (more safer) and refresh your inventory like crazy...	Evil Poogle Plushie Avatar Have the Malevolent Sentient Poogle Plushie (TCG) Item in your inventory and refresh your inventory!
Hungry Skeith Avatar Feed any item with the word 'Skeith' in it to a Skeith	Jhuidah Avatar Bring a Tabogo Potion to Jhuidah, who is trapped in the fish bowl. This avatar can no longer be gained.
Coconut JubJub Avatar Feed your Coconut JubJub an item with the word coconut in it	Chokato Avatar Have the Chokato (TCG) item in your inventory and refresh your inventory multiple times.
Heermeedjet and Meerouladen Avatar Gain Heermeedjet and Meerouladen as battledome challengers by refreshing the Trap Door. Then defeat them at the battledome and report back to Defenders of Neopia Headquarters for one of the two avatars	Heermeedjet and Meerouladen Avatar Gain Heermeedjet and Meerouladen as battledome challengers by refreshing the Trap Door. Then defeat them at the battledome and report back to Defenders of Neopia Headquarters for one of the two avatars
I'm Smelly Avatar Hold 10 items with the word 'dung' in your inventory and continuously refresh your inventory	Grarrl Warrior Avatar Get the Grarrl Gladiator Card and you will have the avatar.
Knight Avatar If you fought for Meridell in the Meridell War, refresh at your user lookup.	Darkling Avatar If you fought for Darigan in the Meridell War, refresh at your user lookup.
Plushie Eyrie Squawk Avatar Type squawk in the neoboards. Then go to chat preferences and search for the Plushie Eyrie Squawk Avatar in the drop-down list.	Freaked Korbat Avatar You get a highscore in top 50 of Korbat's Lab.
Werelupe Avatar You have to own a halloween lupe and then go to its lookup.	Jetsam Chomp Avatar Feed a pepito to your Jetsam.
I Saved Mystery Island Avatar You must have been within the first 10,000 to successfully solve the mystery plot.	Volcano Reward Avatar You must have been within the first 100 to solve the mystery plot.
Fuzzle Boo Avatar Keep playing with an Evil Blue fuzzle until it appears.	Edna Cackle Avatar Complete Edna's Quest and there's a random chance to get it.
Extreme Potato Counter Avatar Get a high score (trophy) in Extreme Potato Counter.	Avatar Collector Avatar Be in the Avatar High Score List.
Deadly Dice Avatar Win at Count Von Roo's [Deadly Dice] game.	Mystery Island Stamp Avatar Complete the Mystery Island page of the Stamp Album.
Black Pteri Caw Avatar Gain the Back Pteri as a battledome challenger (random event) and defeat him in the battledome.	Haunted Woods Stamp Avatar Complete the Haunted Woods page of the Stamp Album.
Plushie Tycoon Avatar Receive a high score (trophy) in Plushie Tycoon.	Tyrannian Stamp Avatar Complete the Tyrannian page of the Stamp Album.
Virtupets Stamp Avatar Complete the Virtupets page of the Stamp Album.	Plushie Draik Avatar Unreleased.
Illusen Avatar Unreleased.	Earth Faerie Avatar Unreleased.
Darigan Angry Avatar Unreleased.	Seasonal Attack Pea Avatar Unreleased.
Spardal Woof Avatar Unreleased.	Nimmo Relax Avatar Unreleased.