The mob spawn items can be found in your creative inventory. Most look like shields with different designs on them and the boss spawn items look like images of the bosses themselves. You can use these spawn items to spawn mobs into the game. While holding the item, right-click on the ground and the mob will appear. If you left click on a mob while holding a spawn item it will save the entities data to the spawn item. Then the next time you use the spawn item the mob that is created will have the same data, like weapons and potion effects.
The NPC Spawn Item and boss spawn items have a special function. If you left-click on two mobs while holding one of these spawn items it will make the mobs fight. This is mostly useful for making bosses fight or starting fights between mobs from different mods. Chocolate Quest mobs will not fight members of their own faction but will readily fight members of different factions without being prompted. For example, a Skeleton will not fight a Zombie because they both belong to the mob_undead faction. But a skeleton will immediately attack a Gremlin because it is a member of a different faction.