A Monster is usually any non-human being that the player character can interact with (attack, speak with, etc). It is also used in the old D&D sense of any NPC, human or not. Traditionally, most monsters are represented by the letters of the alphabet, although mimics appear as the symbol for whatever they are mimicking and '&' is often used for demons. In games using tile graphics, they are represented by pictures of the monsters or at least something the developer figured was close enough from some existing tile set. Killing a monster is the most usual way to gain experience and they are also the source of most treasure. Monsters may have an inventory and use (some subset) of the items in the game, or they may simply drop some items on death.
The term "monster" is considered offensive to some of the dungeon inhabitants, who prefer the term "underground security and quest assistance worker".
One question that some developers have is "How do you come up with names for monsters?" Now, if you have a small number of monsters, you may just use a few well-known mythological or fictional creatures with easily-recognized names. Not terribly interesting, but easy and quickly establishes what the player is dealing with.
What if you want to be more creative/original with your monster names?
Okay, basic techniques:
Really strange stuff can get exotic mixes of letters: volfindul, gostiburr, xontl, but this should generally be reserved for very exotic things -- weird monsters or aliens.
I'll ignore use of a constructed language, as this is a whole article in itself, but suffice to say that if you have a conlanguage as part of your setting, it can be used for naming monsters.
Using terms from another language than the game uses is another strategy; this is an easy way to make "exotic" sounding monsters. This is somewhat similar to the conlanguage approach, but uses an existing natural language. You don't really even need to be fluent in the other language, just steal some names.
Finally, there's the basic adjective + noun approach. And here are monster names I've yet to see in a roguelike game:
- gilded demon
- wheeled demon
- radioactive serpent
- spineless fiend
- horned cat
- flying blob
- hopping killer
- flailing lizard
- one-eyed troll
- stinking worm
- boiling demon
- oozing beetle
- mind moth
- brain slurper
- phantom ogre
- leaping dragon
- library goblin
- acidic vole
- troglodytic eagle
- festering cow
- heavily-armed moron
- slinking shadow
- moon crow
It is possible to use double adjectives allow for more variation. Likewise, one can add suffixes like "of the Deep", "with a Wart" or "from Boston". But watch out for overly long names that make your messages less readable. A "gnarled demonic pangolin of the Burning Wastes" has a pretty cool name, but do you really want to see something that long in your messages?
One must take care to take an appropriate tone. Even following the adjective, adjective, noun formula the same creature can be named for quite different feels. Compare "cute little kitten" and "ferocious young leaper".
I made up the above list on the spot, throwing out a very few that I thought were too stupid. Generally speaking, I think a name should serve as a starting place, used as inspiration for creating the actual monster. You can generate names randomly, pairing adjectives and nouns from whatever lists you care to use. Then, however, you need to decide which are actually useable. Those with truly powerful script fu can have some names generated along alternative templates, such as double adjective noun ("flaming sky clown") or double noun ("tiger shark").
For example, let's take "moon crow". I like the name, but now I need to create a monster that deserves it. A physical description might be: "A large crow with an eerie pale glow that swells and fades as you watch." It has basic crow features: it flies, it is a small creature, it is a natural creature (specifically a bird, if the game cares about that), it can peck with its beak. I might let it scratch with its claws as well. It is smart for an animal and since it is a monstrous version, I might make it fully intelligent. The moon suggests powers of light and darkness, so give it the ability to cast Light and Darkness spells and the ability to see in the dark. The moon is also associated with madness, so it can cast Confusion, Depression, and Hallucination. Like normal crows, it appears in flocks.