ModEventBusClientEvents

@Mod.EventBusSubscriber(modid = OceanicExpansion.MODID, bus = Mod.EventBusSubscriber.Bus.MOD, value = Dist.CLIENT)
public class ModEventBusClientEvents {
    @SubscribeEvent
    public static void registerLayer(EntityRenderersEvent.RegisterLayerDefinitions event){
        event.registerLayerDefinition(ModModelLayers.TIAMAT_LAYER, TiamatModel::createBodyLayer);
    }

}

ModEventBusEvents

@Mod.EventBusSubscriber(modid = OceanicExpansion.MODID, bus = Mod.EventBusSubscriber.Bus.MOD, value = Dist.CLIENT)
public class ModEventBusEvents {
    @SubscribeEvent
    public static void registerAttributes(EntityRenderersEvent.RegisterLayerDefinitions event){
        event.registerLayerDefinition(ModModelLayers.TIAMAT_LAYER, TiamatModel::createBodyLayer);

    }
}

ModModelLayers

public class ModModelLayers {
    public static final ModelLayerLocation TIAMAT_LAYER = new ModelLayerLocation(
            new ResourceLocation(OceanicExpansion.MODID,"tiamat_layer"),"main");

}

TiamatEntity

public class TiamatEntity extends Animal {

    public TiamatEntity(EntityType<? extends Animal> pEntityType, Level pLevel) {
        super(pEntityType, pLevel);
        this.moveControl = new SmoothSwimmingMoveControl(this, 85, 10, 0.02F, 0.1F, true);
        this.lookControl = new SmoothSwimmingLookControl(this, 10);
    }

    @Override
    protected void registerGoals() {
        this.goalSelector.addGoal(0,new RandomSwimmingGoal(this,10,1));
        this.goalSelector.addGoal(1,new BreedGoal(this,5));
        this.goalSelector.addGoal(2,new TemptGoal(this,5, Ingredient.of(Items.ENCHANTED_GOLDEN_APPLE),false));
        this.goalSelector.addGoal(3,new FollowParentGoal(this,6));
        this.goalSelector.addGoal(4,new LookAtPlayerGoal(this, Player.class,3f));

    }
    public static AttributeSupplier.Builder setAttributes(){
        return Animal.createLivingAttributes()
                .add(Attributes.MAX_HEALTH,1)
                .add(Attributes.MOVEMENT_SPEED,0.2D)
                .add(Attributes.ARMOR_TOUGHNESS,0.5D)
                .add(Attributes.ATTACK_KNOCKBACK,0.2D)
                .add(Attributes.KNOCKBACK_RESISTANCE,3D)
                .add(Attributes.ATTACK_DAMAGE,5f);
    }


    @Nullable
    @Override
    public AgeableMob getBreedOffspring(ServerLevel pLevel, AgeableMob pOtherParent) {
        return ModEntities.TIAMAT.get().create(pLevel);
    }

    @Override
    public boolean isFood(ItemStack pStack) {
        return pStack.is(Items.ENCHANTED_GOLDEN_APPLE);
    }

}

TiamatModel

public class TiamatModel<T extends Entity> extends HierarchicalModel<T> {
	// This layer location should be baked with EntityRendererProvider.Context in the entity renderer and passed into this model's constructor
	public static final ModelLayerLocation LAYER_LOCATION = new ModelLayerLocation(new ResourceLocation("modid", "unknown"), "main");
	private final ModelPart tiamat;
	private final ModelPart head;

	public TiamatModel(ModelPart root) {
		this.tiamat = root.getChild("tiamat");
		this.head = tiamat.getChild("upperbody").getChild("head");
	}

	public static LayerDefinition createBodyLayer() {
		MeshDefinition meshdefinition = new MeshDefinition();
		PartDefinition partdefinition = meshdefinition.getRoot();

		PartDefinition tiamat = partdefinition.addOrReplaceChild("tiamat", CubeListBuilder.create(), PartPose.offset(0.0F, 21.0F, 0.0F));

		PartDefinition upperbody = tiamat.addOrReplaceChild("upperbody", CubeListBuilder.create(), PartPose.offset(0.0F, 0.0F, 0.0F));

		PartDefinition head = upperbody.addOrReplaceChild("head", CubeListBuilder.create().texOffs(25, 27).addBox(-2.0F, -2.0F, -1.0F, 4.0F, 4.0F, 7.0F, new CubeDeformation(0.0F))
		.texOffs(12, 36).addBox(-1.0F, -2.0F, 6.0F, 2.0F, 2.0F, 8.0F, new CubeDeformation(0.0F))
		.texOffs(18, 24).addBox(-1.0F, -1.0F, 13.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F))
		.texOffs(18, 24).addBox(-1.0F, -1.0F, 12.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F))
		.texOffs(18, 24).addBox(-1.0F, -1.0F, 11.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F))
		.texOffs(18, 24).addBox(-1.0F, -1.0F, 10.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F))
		.texOffs(18, 24).addBox(-1.0F, -1.0F, 9.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F)), PartPose.offset(0.0F, 0.0F, 8.0F));

		PartDefinition jaw = head.addOrReplaceChild("jaw", CubeListBuilder.create().texOffs(0, 34).addBox(-1.0F, -1.0F, 0.0F, 2.0F, 2.0F, 8.0F, new CubeDeformation(0.0F))
		.texOffs(18, 19).addBox(-1.0F, -2.0F, 7.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F))
		.texOffs(18, 19).addBox(-1.0F, -2.0F, 6.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F))
		.texOffs(18, 19).addBox(-1.0F, -2.0F, 5.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F))
		.texOffs(18, 19).addBox(-1.0F, -2.0F, 4.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F))
		.texOffs(18, 19).addBox(-1.0F, -2.0F, 3.0F, 2.0F, 2.0F, 0.0F, new CubeDeformation(0.0F)), PartPose.offset(0.0F, 1.0F, 6.0F));

		PartDefinition tongue = jaw.addOrReplaceChild("tongue", CubeListBuilder.create().texOffs(16, 2).addBox(-1.0F, 0.0F, -2.0F, 2.0F, 0.0F, 7.0F, new CubeDeformation(0.0F)), PartPose.offset(0.0F, -1.0F, 3.0F));

		PartDefinition body = upperbody.addOrReplaceChild("body", CubeListBuilder.create().texOffs(0, 0).addBox(-4.0F, -3.0F, -3.0F, 6.0F, 6.0F, 11.0F, new CubeDeformation(0.0F)), PartPose.offset(1.0F, 0.0F, 0.0F));

		PartDefinition fin1 = body.addOrReplaceChild("fin1", CubeListBuilder.create(), PartPose.offset(-5.0F, 3.0F, 1.0F));

		PartDefinition fin1_r1 = fin1.addOrReplaceChild("fin1_r1", CubeListBuilder.create().texOffs(0, 20).addBox(0.0293F, -1.1866F, -0.5985F, 1.0F, 6.0F, 3.0F, new CubeDeformation(0.0F)), PartPose.offsetAndRotation(0.0F, -2.0F, 0.0F, -0.8895F, 0.2054F, 0.2406F));

		PartDefinition fin2 = body.addOrReplaceChild("fin2", CubeListBuilder.create(), PartPose.offset(3.0F, 3.0F, 1.0F));

		PartDefinition fin2_r1 = fin2.addOrReplaceChild("fin2_r1", CubeListBuilder.create().texOffs(0, 0).addBox(-1.0293F, -1.1866F, -0.5985F, 1.0F, 6.0F, 3.0F, new CubeDeformation(0.0F)), PartPose.offsetAndRotation(0.0F, -2.0F, 0.0F, -0.8895F, -0.2054F, -0.2406F));

		PartDefinition lowerbody = tiamat.addOrReplaceChild("lowerbody", CubeListBuilder.create(), PartPose.offset(0.0F, 0.0F, 1.0F));

		PartDefinition midsegment = lowerbody.addOrReplaceChild("midsegment", CubeListBuilder.create().texOffs(22, 5).addBox(-2.0F, -2.0F, -15.0F, 4.0F, 4.0F, 12.0F, new CubeDeformation(0.0F)), PartPose.offset(0.0F, 0.0F, 0.0F));

		PartDefinition afin1 = midsegment.addOrReplaceChild("afin1", CubeListBuilder.create(), PartPose.offset(-3.0F, 2.0F, -11.0F));

		PartDefinition afin1_r1 = afin1.addOrReplaceChild("afin1_r1", CubeListBuilder.create().texOffs(12, 34).addBox(1.0F, -1.0F, -2.0F, 1.0F, 5.0F, 3.0F, new CubeDeformation(0.0F)), PartPose.offsetAndRotation(-1.0F, 0.0F, 1.0F, -1.0908F, 0.0F, 0.0F));

		PartDefinition afin2 = midsegment.addOrReplaceChild("afin2", CubeListBuilder.create(), PartPose.offset(3.0F, 2.0F, -11.0F));

		PartDefinition afin2_r1 = afin2.addOrReplaceChild("afin2_r1", CubeListBuilder.create().texOffs(0, 34).addBox(-2.0F, -1.0F, -2.0F, 1.0F, 5.0F, 3.0F, new CubeDeformation(0.0F)), PartPose.offsetAndRotation(1.0F, 0.0F, 1.0F, -1.0908F, 0.0F, 0.0F));

		PartDefinition tail = lowerbody.addOrReplaceChild("tail", CubeListBuilder.create().texOffs(0, 17).addBox(-1.0F, -2.0F, -14.0F, 2.0F, 3.0F, 14.0F, new CubeDeformation(0.0F)), PartPose.offset(0.0F, 0.0F, -15.0F));

		PartDefinition tailfluke = tail.addOrReplaceChild("tailfluke", CubeListBuilder.create(), PartPose.offset(0.0F, 0.0F, -13.0F));

		PartDefinition tailfluke_r1 = tailfluke.addOrReplaceChild("tailfluke_r1", CubeListBuilder.create().texOffs(18, 8).addBox(0.0F, 5.5F, -0.4019F, 0.0F, 3.0F, 13.0F, new CubeDeformation(0.0F)), PartPose.offsetAndRotation(0.0F, 0.0F, -12.0F, 0.5236F, 0.0F, 0.0F));

		PartDefinition tailfluke_r2 = tailfluke.addOrReplaceChild("tailfluke_r2", CubeListBuilder.create().texOffs(0, 11).addBox(0.0F, -7.0F, 2.0F, 0.0F, 3.0F, 6.0F, new CubeDeformation(0.0F)), PartPose.offsetAndRotation(0.0F, 0.0F, -9.0F, -0.5236F, 0.0F, 0.0F));

		return LayerDefinition.create(meshdefinition, 64, 64);
	}

	@Override
	public void setupAnim(T entity, float limbSwing, float limbSwingAmount, float ageInTicks, float netHeadYaw, float headPitch) {

	}

	@Override
	public void renderToBuffer(PoseStack poseStack, VertexConsumer vertexConsumer, int packedLight, int packedOverlay, float red, float green, float blue, float alpha) {
		tiamat.render(poseStack, vertexConsumer, packedLight, packedOverlay, red, green, blue, alpha);
	}

	@Override
	public ModelPart root() {

		return tiamat;
	}

TiamatRenderer

public class TiamatRenderer extends MobRenderer<TiamatEntity,TiamatModel<TiamatEntity>> {
    public TiamatRenderer(EntityRendererProvider.Context pContext) {
        super(pContext, new TiamatModel<>((pContext.bakeLayer(ModModelLayers.TIAMAT_LAYER))), 2f);
    }

    @Override
    public ResourceLocation getTextureLocation(TiamatEntity tiamatEntity) {
        return new ResourceLocation(OceanicExpansion.MODID,"textures/entity/tiamat3.png");


    }

    @Override
    public void render(TiamatEntity pEntity, float pEntityYaw, float pPartialTicks, PoseStack pMatrixStack, MultiBufferSource pBuffer, int pPackedLight) {
        pMatrixStack.scale(5f,5f,5f);
        if(pEntity.isBaby()){
            pMatrixStack.scale(0.1f,0.1f,0.1f);
        }
        super.render(pEntity, pEntityYaw, pPartialTicks, pMatrixStack, pBuffer, pPackedLight);
    }
}