Rogue and some of Jeff Lait's 7DRLs have simple rule: one creature for each letter.
Yes, monsters are usually grouped by type in roguelikes, not by their first letters.
Are there any examples of choosing letters based on something else than the first letter of a monster's name or type name? I think I've seen somewhere something about choosing a letter which looks similar, like choosing S for S-shaped beasts and Y for horned beasts. Well, Crawl does these two, but I am not sure whether this is the reason.
And demons in Larn are denoted with digits. And named something like "second type demon", etc.