The ore, natch. Zombies... OK, could be parallel to iron ingots, but it seemed to happen rather more often. But... chickens? (mysticalagriculture[1.11]-1.3.6.jar)

As I said you can disable it in the config. I made mobs drop Inferium because you generally need quite a lot of it. It's also an important way to get Inferium in certain types of modpacks (like skyblocks).

You can lower or disable the drop chances in the config.

Looking at that... I note that passive mobs actually have a higher drop chance than hostile ones.

In general, I assume a mod's default represent how the maker chose to balance the mods, I just found this rather odd.

Whats wrong with that?

Inferium, Inferium everywhere!

Seriously, mobs normally drop items specifically associated with them -- pieces of the mob, items they were carrying, items reflecting their appearance (e.g., ghast tears) or role (evoker totems). IIRC, the biggest stretch in vanilla is the zombie rare drops; but then, zombies conceptually come out of the ground (graves), so it's plausible they might have a carrot or potato, and they have armor and weapons from somewhere, so it's reasonable they could have an iron ingot from the same source.

But why would all monsters, much less all farm animals, potentially have pieces of inferium?