In most big roguelikes each letter maps to a class of monsters, for example in ADOM: e-eyes, S-spiders, s-snakes, i-insects, h-other humanoids, F-fantastic beasts. Specific monsters in a given group are denoted with colors. You can adjust it to your needs (ADOM has separate letters for (T)rolls, (o)rcs, dark elves (u), and townspersons (t), but elves, dwarves and hurthlings all use "h"; if you game featured e.g. more types of elves and no eyes, then (e) would be used for elves).