I saw this post and realized that this has been my problem all along. My mod was at a standstill. Thanks! Btw, I tried using this method for the entity register and spawn:
public void load()
ModLoader.registerEntityID(EntityCustomCow.class, "CustomCow", -127);
ModLoader.addSpawn(EntityCustomCow.class, 15, 3, 10, EnumCreatureType.creature);
Apparently, Id -127 is not taken, so if you're planning on only adding only one mob to the game, use that number. I'm not sure if this fix is very reliable, but it kept me from recoding everything and it works (I know because I tested it in-game). Hopefully there will be an eventual fix, but for now I'm gonna use -127.
Thanks for the help