So you’ve decided to make a minecraft mod and wanted to add a new creature into the world. In this case it’s going to happen when you right click an item though this code would work form anywhere.

public ItemStack onItemRightClick(ItemStack itemstack,World world,EntityPlayer player) {
   EntityChicken c = new EntityChicken(world);
   c.setPosition(player.posX, player.posY, player.posZ);
   return itemstack;

Simple as that.