LineageCore is a mod I made specifically to create Species that work with all kinds of things.

Whether you want to use Java or purely JSON, you can make a species with either.

Here's some basic guides in the form of an example json file that you should put into ".minecraft/lineages/template.json" or ".minecraft/lineages/template/template.json"
NOTE: The artDirectory searches ONLY in the lineages folder, nowhere else.

Display Name: Changes the title of your Species/Subspecies
Species Description: Your Description
Species ID: Your Species ID, "id" denotes a core species and "id_sub" denotes a subspecies,.
Order [OPTIONAL]: Changes the order of where your species appears, 100 is default and will make it appear right-most.
Has Subspecies [OPTIONAL]: True of False, should be able to be used with subspecies as well.
Is Subspecies Of [OPTIONAL]: Denotes what species it is a subspecies of, please do not use this if your species is a core species. "human" and "normal_human" are the base core species.
Art Directory [OPTIONAL, BUT RECOMMENDED]: Denotes where the art is, searches the Lineage Folder and any Subfolders.
On Select [OPTIONAL]: In Java, it only supports Java Code, obviously. In Json, it only supports commands as shown below (For Safety Reasons).

{
  "displayName": "Template Species",
  "speciesDescription": "This is an example species loaded dynamically from the lineages folder.",
  "speciesID": "template_species_sub", 
  "order": 100,
  "hasSubspecies": false,
  "isSubspeciesOf": "human",
  "artDirectory": "template/template_art.png",
  "onSelect": {
    "type": "command",
    "command": [
      "/give @s minecraft:bow 1",
      "/give @s minecraft:arrow 64"
    ]
  }
}

Java Code Example:

public static final RegistryObject<Species> ELF = SPECIES.register(
        "elf",
        () -> new Species(
                "Elf",
                "Agile archers attuned with nature.",
                "elf",
                true,
                player -> {
                    // Example 1: Add potion effects directly
                    player.addEffect(new MobEffectInstance(MobEffects.MOVEMENT_SPEED, 999999, 0, false, false));
                    // Example 2: Give an item directly
                    player.getInventory().add(new ItemStack(Items.BOW));
                    // Example 3: Send a message or play a sound
                    player.sendSystemMessage(Component.literal("You have attuned with nature!"));
                },
                null,
                -1
        )
);

Made by Hivemere, aka me.

[FORGE ONLY - 1.20.1]