User blog comment:Mathmagician/Project - Individual Pages/@comment-1863391-20120111114347/@comment-4674838-20120112023743

Here how I think about it:

Definition: A player-character is a living entity that is controlled by a player.

Definition: An NPC is a non-player-controlled living entity that 1) is friendly towards the player and 2) can truly be considered a "character," meaning that it is representative of a person or bears a sufficient amount of similarity to a human being.

Definition: A Monster is any living entity that is neither a player-character nor an NPC.

That is, monsters are living entities which are hostile to humans, or they are quite dissimilar from humans, or possibly both. The goldfish would then be a monster. An argument could be made that we should distinguish friendly monsters from enemy monsters by calling the friendly ones "creatures." While I like this idea in theory... in practice there would be so few creatures in the game (birds, bunnies, goldfish... that's it?) that it seems silly to make a new class specifically for them. And, ultimately, creatures would just be a subclass of monsters anyway, since they're non-human. So, either way, in my opinion the goldfish is a monster.