REPLY TO DESH

no. each enemy has a defined probability to drop an item. Those probabilities are:
0% (like most bosses and some enemies)
5% (like most enemies)
25% (looks like only Horseman has it)
100% (like Dr.Lugae to give a Tower Key at the end of the fight)

If the enemy did drop something, the game follows the link to the list of ItemDrops, where a set of four items it given. The probabilities for each different items were given in my previous post.

Hope my reply was interesting ^-^

ps. btw, it's Zyrthofar, not Zrythofar