I want to make a copy of the cow but bigger. But when i am trying to summon it in the game there is a error "failed to create entity". Also i dont understand what exactlly .build("big_cow") doing 

public static final DeferredRegister<EntityType<?>> ENTITY_TYPES = DeferredRegister.create(ForgeRegistries.ENTITY_TYPES, MyMod.MODID);

public static final RegistryObject<EntityType<Cow>> BIG_COW = ENTITY_TYPES.register("big_cow",
            () -> EntityType.Builder.of(Cow::new, MobCategory.CREATURE)
            .sized(1.5f, 1.5f)
            .build("big_cow"));

public static void register(IEventBus eventBus)
    {
        ENTITY_TYPES.register(eventBus);
    }

 public MyMod()
    {
        IEventBus modEventBus = FMLJavaModLoadingContext.get().getModEventBus();

        MyEntity.register(modEventBus);

        modEventBus.addListener(this::commonSetup);
        MinecraftForge.EVENT_BUS.register(this);
    }

Edited December 2, 20222 yr by Klarks

You're missing the renderer through EntityRenderersEvent$RegisterRenderers, spawn placement through SpawnPlacements#register in the queue within FMLCommonSetupEvent, and attributes added through EntityAttributeCreationEvent. As for the `#build` method, that is for specifying the name of the entity to be referenced in the datafixer if things change across versions and the entity data needs to be adjusted; however, it currently is not supported by Forge.