Animated Corpses, the most basic type of Monster, are usually the first enemy a fledgling adventurer challenges.
Since the conditions for these Monsters to form are simple, they can arise in any obscure corner, and they appear in large numbers, even more resilient than goblins.
Moreover, modifying them is easy, making them a favorite of Necromancers. You can add any attribute you like, such as ice or fire, and if it doesn't suit them, the Animated Corpse will quietly perish on its own.
In general, you can think of Animated Corpses as a magical version of zombies, just without the contagious aspect. The chance of becoming an Animated Corpse from their infection is probably lower than the chance of poisoning yourself with a feces pot.