This post is about resources to help you come up with a concept or set of concepts for your OC. It's also about a couple really good art sites about OCs (other than TH of course). There's countless development resources out there, so I will not be featuring those.
This list also includes sites where you can use your character! Please suggest things for me to add.
I was not asked to advertise anything in particular. I just want to promote these sites and ideas.

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Art Fight

🤖 It's an art trading game that takes place every July all month long. It's a great way to recieve gift art and find characters to draw. I've been a member since 2017 and every year has been amazing.
🤖 Click here to learn more about how it works.
🤖 I recommend submitting your characters and finding other characters to draw before July.
🤖 Check in during June/late June to pick your team. If you don't pick a team in advance your team will be picked randomly. You have limited re-rolls to change it.
🤖 You can get achievements for drawing a lot and for picking your team. Find all of the achievements here.
🤖 Follow them for updates: Twitter | Instagram | Discord
🤖 Their Discord hosts it's own art events.

PaperDemon

😈 A free ARPG game about your OCs. This site is extremely inclusive of neurodivergent and LGBTQ+ people.
😈 All prompts can be completed using visual art or writing.
😈 Make sure to read their character rules before submitting your character.
😈 After your character is accepted you will need to draw a tailsman for them, and then they'll be given a random element.
😈 After this, you'll draw/write about your character using their magic. If you want to change your OC's element, there's a prompt for it!
😈 After finishing those prompts you'll learn to obtain and craft items.
😈 You can earn rewards for posting comments and on the forum.
😈 If you draw someone else's character (with permission) you'll both earn specific rewards.
😈 New prompts are posted frequently!
😈 These Open Portals feature prompts with no time limit.

Collaborative Forums For Character Creation

🏘 Some of these are dead and can be added on. If you want to make a new post, please credit the original. Also let me know so I can promote it here! Some of these are still actively being used. All of them are really good ideas!
🏘 Create a Fanchild for the person above you
🏘 Create a Song-Inspired Character for the Person...
🏘 Create A Backstory For the Character Above You
🏘 Create parents for the OC above!
🏘 Create a character for the person above you!
🏘 Create a Fan OC for the Person Above!

General Character Prompts

💥 Roll a dice to pick various OC elements. You can roll a dice with Google.
💥 https://artprompts.org/character/
💥 #Oc-tober
💥 https://drawthisshitt.tumblr.com/post/169265906134/character-design-inspirationchallenge
💥 OC Ideas Wheel
💥 How to create/improve your species by ImperfectEnthusiast
💥 Make an OC using a specific color palette
💥 Let's make an oc meme
💥 Let's make an oc meme (alternate version) by @fclkly
💥 More versions of the let's make an oc meme
💥 Species from across the fantasy genre
💥 Sonas Masterlist

Furry Prompts

😺 RANDOM FURRY GENERATOR by blancblackcastle
😺 Fursona Generator by aaron
😺 Species Wheel
😺 Fursona Name Generator

Fantasy Character Prompts

💫 FANTASY CREATURE GENERATOR
💫 Monster Girl Maker 1 & 2 (both are detailed dress up games)
💫 Who the **** is my #DND character?
💫 Let's make a DnD character meme by

Fandom Character Prompts

🐧 Various Furby Resources (such as dress up games, coloring pages, tutorials)
🐧 Homestuck Fantroll Generator by Kintupsi (download the file, open the HTML file, then click "hatch my troll")
🐧 Epithet Erased OC Ideas by team-sadbabies
🐧 Animal Crossing Villager Ideas
🐧 Let's make a Fire Emblem oc meme
🐧 (Must be logged into Neopets to view) A list of Neopets dragon species, breath types, abilities, and forms to help you create a dragon.
🐧 #PaintbrushOCs - Make an OC based on a Neopets paint brush or paint one of your OCs.
🐧 Make a character using an established species from media. Examples: Furby, Chao, Pokemon, and Neopets

My Prompts For You

🎩 Combine traits from various generators. Try prompt wheels! You can find really specific ones by Googling "trait type name generator". Character generators are popular on Tiktok too. They come with multiple traits for one OC. This one provides a bit of lore!
🎩 Use an obscure animal
🎩 Look at your TH favs and combine lots of small details and traits you like
🎩 Doodle a weird creature without prior planning
🎩 Start with a unique piece of clothing
🎩 Start with one of your niche interests
🎩 Start with an aesthetic
🎩 Use a pride flag's color palette
🎩 Use a logo's color palette

Original Species

👾 Species Discussion on TH
👾 🌐 Original Species Directory 🌐
👾 ⭑Open Species List⭑
👾 New Open Species Discord Servers
👾 Open Species on dA
👾 Use Lorekeeper's template to create a site for your species

Character Design Tools (such as dress up games)

👖 Make a 3D model of your D&D Character - Hero Forge
👖 Dress up a penguin from Club Penguin - Club Penguin Mountains
👖 Make a detailed anime character if you play - Gaia Online
👖 Color a Furby Online - supercoloring.com
👖 Monster Girl Maker 1 & 2
👖 A gold mine of user made dress up games (including various fandoms) - Picrew
👖 3D Chao Maker
👖 Design a Neopet - Dress To Impress
👖 League Of Legends Avatar Creator
👖 Lofigirl Generator - Surprisingly inclusive!