near the swallow mane there was some miasma/
Probably just another cave creature that died of old age.
The "deaths" thing is what usually triggers a name, but it's hardly necessary.
Any "monster" that the dwarves see regularly can get a name.
If a monster kills dwarfs (or even dogs?) it will probably get a name.
If one of your dogs kills some monsters it might get a name.
I believe it's mostly for color - no significant effect on game play.
Wow, for once you're not talking complete shit. Should point out that a dog will always get a name after it has killed one thing (be it a goblin, kobold or even those thieving little rhesus macaques), it's not a case of "It might get a name if it's had a few kills".
And for creatures that have been given a name as a result of killing something, it does have an effect on gameplay because that creature will now be listed in Legends mode (if that creature is female and can breed, any children it gives birth to after it's made a kill will also appear in Legends mode (even in the likely event that they're unnamed)), and there's a chance of engravers carving images of that creature striking down one of its victims.
I don't think this is the case for creatures given a name just because they've been in the local area for a certain period of time (dwarves seem to like naming all the ___men in chasms and stuff, even if they haven't done squat)