All the monsters: N.
N is a dangerous letter. Nastiness, nihilism, nothingness, and the night all start with "N". There are no giant animals, amiable bumblers, or typical PC races under "N". Even the nymph is, as wrtten, one of the deadliest things in the book.
"Naga are snake-like creatures with good brains and magical abilities". (Good brains?)
In real life, nagas are still very important in parts of India. I find the idea of humanoid creatures worshipping a naga (an "ordinary" naga--not some ethereal naga god) interesting since--assuming the naga has a range of desires and interests broadly equivalent to an ordinary aristocratic human--then a naga is esentially a kind of royal invalid. Unlike a dragon or something, the naga might actually need its worshippers--"Charles, would you mind turning the pages of this book for me, I've dreeadfully tired of these papercuts on my nose"--and risks becoming pathetic without them.
I suggest that the otyugh is a tedious, uncompelling, and redundant monster unless you have a really cool and gross little miniature of it, in which case it can make a very pleasant unpleasant pulp diversion. The neo-otyugh doesn't even have that going for it.
So there's a hag meaning like just an old witch and there's a hag meaning like a sort of stringy gross giantess, and there's a zone in between. Either way the hag is always a GMIWNLF. That thing Jack Nicholson makes out with in The Shining also partakes somewhat in the horror of hagness.
There's some pop psychology quiz where they ask you how you'd react to seeing a naked member of the opposite sex that's 50 years older than you. The answer is supposed to be how you feel about death.
This is thinking about death in a different way than the way the undead make you think about death. The skeleton's about simply being gone--simply not being there any more. The hag is about all the humiliating, pitiless, flabby, gooey things that are going to happen to you on the way to bodily extinction. Horror of age, the body, the skin, physical need. There's a reason they're always cooking.
It is a mare of the night. I feel like the nightmare as presented is a little too big on bluster .
I mean. look at a hell hound--what's it going to do? It's gonna leap through the air with its paws on fire and land on your throat and chew on you. The nightmare? Not so much. And while it is true that horses bite hard, the symbolic point of the nightmare is that it's just something for something considerably more badass to ride around on.
You don't need a nightmare. If you go look at the cover of Death Dealer that guy's just riding a horse. It's a big, black, tough, scary-looking horse but it doesn't have webbed ears. Likewise, the steeds upon which The Nine ride in the movie are just horses.
Horses have inherently understated faces--their eyes are looking down at what they are doing--they concern themselves with chewing up ground and leave the rest to you. Horses with wide eyes or crazy expressions usually just look like they are doing their job wrong. If you really can't do without a flamboyantly menacing transport I suggest a skeletal steed.
Apparently "nixies delight in enslaving humans" and also apparently a nixie won a gold medal in the Olympic 40 meter freestyle and had its smiling, waving portrait taken for the cover of a Wheaties box and also apparently a black and white version of this picture graces the otherwise very menacing entry in the Monster Manual.
Since they are supposed to appear in lakes rather than the ocean, the idea I guess is that Nixies are supposed to be frightening aquatic fairy folk of the indifferent-to-human-morality-variety. They need a better pr department.
The nymph both has higher standards than the dryad (it takes a male with an 18 rather than a 16 charisma to catch her eye) but she's also less desperate (the dryad steals desirable men away whereas the nymph merely has a chance of being "favorably inclined" towards the person.)
Mechanically, the nymph as presented is little more than a trap--you have a pretty good chance of dying or going blind just from looking at her and if you manage to pass your saving throw she'll probably unleash some 7th level druid trouble on you just for coming near her house.
The nymph represents a common Monster Manual solution to making good creatures interesting, that is: find a way to make them hostile.
It seems like playing a nymph as a Gandalf-esque, charmingly manipulative schemer rather than some sort of fragile woodsy xenophobe is the way to go. And, needless to say, in my campaign they're pretty much useless unless they can get over their strict heterosexuality.