TribeAPI is a BepInEx plugin that allows for easy addition of custom tribes. To use TribeAPI in your mod, just add it as a project reference, and do something like this:
// if insertYourTribeIconHere is null, the tribe will not show on the card (just like the squirrel tribe). if appearAsTribeChoice is false, the tribe won't appear as a possible choice in the "tribe card selection" event.
// tribeChoiceCardBack can be null if appearAsTribeChoice is false.
NewTribe yourTribe = new NewTribe(insertYourTribeIconHere, appearAsTribeChoice, tribeChoiceCardBack);
Then to actually use the tribe on a card, just use yourTribe.tribe in the tribe list for the card.
To use TribeAPI, put TribeAPI.dll inside of your BepInEx plugins folder. It also probably works for Thunderstore Mod Manager or r2modman but I don't know for sure (I don't use them)
NOTE: This mod was developed for Kaycee's Mod beta, but it should work for the normal game.