All of the monsters: R.
The smoking jacket is the key. Without it, it's just one more organic impediment with a zoomorphic head. Strangely, the game often ignores the smoking jacket and all it implies and treats the Rakshasa as just one more high-level nasty. A recent WotC product has a beholder summoning one as a kind of bodyguard.
A quick perusal of the wikipedia finds no mention of the tiger head at all--the Rakshasa is just a sort of catch-all magic cannibal demon. The tiger head,gives the D&D Rakshasa a distinct identity: tigers have both the coiled poise of housecats and the pimpiness of their striped orange coats. Tigers seem to be saying "I know you know I'm right here, and I know that you know that I know that there's nothing you can do it about it. Because I'm a fucking tiger." Ferocity and royal ease. This eastern demon is not a tempter but a tyrant. Shere Khan acts like Shere Khan for a reason.
Slightly more dangerous and slightly less interesting than Goat, giant.
Rat, giant (Sumatran)
Giant rats appear all the time in dungeons, they're the monster that tells you that you're in a dungeon but you haven't gotten anywhere that anybody else hasn't managed to get yet.
Giant rats do actually exist and--aside from horses--probably therefore qualify as the real animal most likely to appear in Dungeons and Dragons. The giant rats in the Manual are size S which means that they probably aren't yet riding-animal size, but that never stopped people from writing them that way.
What I like about the idea of a goblin or something riding around on a--say--wolf-sized giant rat is the image of the rat getting up on its hind legs and sniffing the air while the goblin scouts around. Perfect thing for going around a dungeon on.
It's strange how modern rays seem. It's difficult to picture anyone before the Sherlock Holmes era having anything to do with them (except maybe in Japan).
Judging from the Monster Manual, Gygax apparently regularly had need of rules to determine what happens when a PC gets swallowed whole by a giant version of a regular animal. He also seems extremely prepared for the prospect of PCs looking for treasure while they are in there. "The manta ray's stomach is the repository of indigestible items--such as the treasure types indicated."
I catch myself wondering if Gygax's original players didn't spend half the Greyhawk campaign just wandering around temperate salt water biomes hoping to get swallowed.
A remorhaz is another one of those sci-fi type monsters that you can't really picture properly unless you've seen an illustration. The job with the remorhaz is to distinguish it from the purple worm and (later) from the frost worm. The only thing that really makes it come alive is the original Trampier picture.
When I was younger I never realized it, but I realize now that he had a peculiar talent for defining things. Whether or not any individual one of his pictures was spectacular or memorable, the Trampier picture always etches the substance and the aura--the idea of the monster--very clearly. I can't immediately think of any comparable artist in that regard in all of art history.
With, for example, Ian Miller or Erol Otus' expressionism you always seem to get more mood (or more Otus, or more Miller) than monster--not that this is a bad thing, it's just that they're more about the image than the thing.
Other artists can define a thing, but in the way a dictionary does--they generally rely so much on realism that it ends up looking like the thing has been observed rather than summoned. A Bosch demon, for example, just looks like he just saw that bug-faced five eyed thing walking around in the Netherlands somewhere and painted it. Durer's work looks like every monstrosity he ever drew resulted from careful scientific study of the beast in its habitat or on a dissecting table. Frazetta seemed to be trying to carefully re-create vividly-colored dreams using all the technical tools at his disposal.
Trampier stylizations, on the other hand, seem to be neither expressions or observation--he looks rather as if he has lived with the legend of the remorhaz as did his father before him and his father before him and so when he's called upon to depict it he gives you a sort of codified but still vital representation of this thing--his work is a sort of pulpy equivalent of the eerily specific monster sculptures produced by early Chinese and Mesoamerican artists with a sort of supreme cultural confidence that the monster has a broken tooth right there and a third arm right there and that you have a sort of sacred responsibility to represent it properly.
Anyway, he makes me want to make the girls want to know what a remorhaz is.
I can't really picture my campaign including a rhinoceros, although I can imagine my PCs seeing a bizarre print on the wall of the library depicting one and wondering what the hell kind of crazy monster that is and wondering whether there is any treasure in its stomach.
See Eagle, giant.
Roper and Rust Monster
I would maintain the roper is superfluous and the rust monster isn't.
Superficially, they have a lot in common: both are original to D&D, both are sort of sci-fi-ish, both have tentacles.
But the roper is basically nothing at all interesting without its tentacles--and so many other things have tentacles. And if they don't then you can easily stick them on.
The rust monster, on the other hand, has a unique ability, is less shapeless in the mind's eye (a roper is "cigar shaped" whereas the rust monster is basically a giant four-legged bug), and the rust monster has personality.
It has personality because while the roper is just one more yellowish brown blob trying to kill you, the rust monster has the subtle agon of all insects--it means you no harm, it's simply gross and in your way and wants to eat something you happen to value.
In an actual fight, the roper is going to lash out and try to eat you and if it doesn't then what's the point? The rust monster, on the other hand, has a million different uses: goblins can prod it toward you or drop it on you, it can simply be in your way minding its own business in a narrow tunnel you need to get through, or hundreds of them can infest a town like rats.
Mandy wants to know if--when you drop them--they helicopter slowly to the floor with their propeller tails.
Now is as good a time as any to mention that when I complained about the gelatinous cube in the G entry, the nice people at Otherworld Miniatures sent me a very nice deluxe cube and also threw in a pair of rust monsters. I'd like to put a picture of them here but I had a hell of a time gluing them together and I wouldn't want my hamfistedness to reflect poorly on them. Anyway, point is they make some pretty good rust monsters over there at Otherworld.
I'm sure there's some story about some circumstance that made this incredibly boring kill-your-players-immediately monster seem necessary when they were originally invented.
I feel like if I was an entrepreneurial lower planes creature aiming for Demon Prince of something, there's unexploited psychobioarchetypal real estate in the Lord of Rot Grubs, Cerebral Parasites, Ear Seekers and Other Creatures That Are Ubiquitous, Almost Undetectable, and Almost Always Fatal niche. Why isn't there that guy? He'd be the boss in no time.
Classically, a revenant refers to someone who's dead, walking around, corporeal, and still in possession of all or most of his or her faculties. So: not a ghost, not a zombie, and not a vampire (the revenant doesn't necessarily have any special powers).
The interesting thing about this kind of revenant is that if it's still walking around and thinking, then what--in practical terms--makes it dead? For this reason, the revenant asks you to define "soul" in one way or another, in order to define, therefore, what it would mean to be "soulless".
In the medieval mind, the soulless were considered animalistic (thus vampires), in modern pop terms we think of soullessness as almost the opposite. "Soulless" suggests to us something Kafkian--without emotion, without appetites or drives, mechanical. Without the passions of the animal.
Strangely, we think of the soul as the animal and the body as mere machine, whereas early Christians though of the body as the animal and the soul as the restraining mechanism.
At any rate, revenants in a campaign that already has ghouls or vampires would be modern Kafkian constructs joylessly going about repetitive rituals or bureaucratic tasks. Better than ghosts, in my humble.