The term monster frequency applies to random monsters generation, as opposed to deliberate polymorph results and more specific ways of monster generation. It refers to a number which can technically range 0-7 (though 6 and 7 are not used) used internally to determine how frequently a monster is generated; it can be thought of as a relative probability that a particular monster will be selected.
When a monster is randomly being selected, the game picks a random monster subject to the following list of conditions with relative probabilities equal to the ratios of numberic monster frequencies (thus a monster with a 4 is twice as common as a monster with a 2); this can be thought of as generating a list of monsters with each monster appearing a number of times equal to its monster frequency and choosing purely randomly off of this list.
Randomly generated monsters must be:
- No greater in difficulty than the average of the level's difficulty and the player level 
- No lower in difficulty than one-sixth that of the current level's difficulty
- Neither unique nor marked as not randomly generated (G_NOGEN) 
- Not genocided or extinct
- If you are on the rogue level:
- Represented by an upper case letter
- If you are in Gehennom:
- If you are not in Gehennom:
- Not marked as only generated in Gehennom (G_HELL) (e.g. hell hound)
The following terms are used to describe the frequency of a monster in the infobox on monsters' pages:
|0||Not randomly generated|