What's In a Name? -- Now Updated For Book 6!

Ragnok - presumably a Gringotts goblin.

Possibly connected to "Ragnarok", an event in Norse myth that marked the final destruction of the world in the conflict between the Aesir and the powers of Hel led by Loki.

Ramora - benevolent silver fish

Similar in name to the "remora", which are fish of the family Echeneidae, having on the head a sucking disk with which they attach themselves to sharks, whales, sea turtles, or the hulls of ships.

Red Cap - dwarflike creatures found wherever blood has been spilled

From Scottish folklore. Red Cap is a thoroughly evil creature. He is a short, stocky old man with long gray hair and claws in stead of hands. He lives on the Scottish Border in ancient ruins of castles, especially in those with a bloody history of war and murder. He owes his name to the fact that he wears a red hat, which is colored by the blood of his victims. Red Cap moves with remarkable speed, despite the fact that he wears iron boots. He can overcome even the strongest man, unless the intended victim remembers to quote a few words from the bible. (Encyclopedia Mythica; thanks to Elizabeth Quest for the tip)

Reducio - counteracts an enlargement charm

"Reduce" means "to bring down in extent, amount or degree; diminish", and comes from Latin "reducere", which means "to bring back; return".

Reductor Curse - blasts solid objects out of one's path

Relashio - sends sparks at an opponent

Remembrall - device that turns red when you've forgotten something

Remus John Lupin - See "Lupin, Remus"

Reparo - repairs broken or damaged objects

Re'em - giant oxen with golden hides

Encyclopedia Mythica: The re'em is a mythological giant. At any given time, only two exist, one male and one female, because had more of them existed, the world could not support them. They are described as fierce, fast, and indomitable. Scholars suggest they could be related to the giant aurochs (Bos primigenius), a species of a wild ox that became extinct during the sixteenth century. On the other hand, he may be a purely mythological creature, based on the bas-reliefs of the huge Mesopotamian and Egyptian beasts.

Rictusempra - Tickling charm

Riddle, Tom Marvolo (Jr.) - birth name of Lord Voldemort.

As he himself points out, "Tom Marvolo Riddle" is an angram of "I am Lord Voldemort". You didn't think Jo could get away with naming a guy "Riddle" without there being something fishy going on!

Riddikulus - transforms a Boggart into a new form comical in appearance

Rita Skeeter - See "Skeeter, Rita"

Ronan - one of the centaurs mentioned in "Philosopher's Stone"

The name "Ronan" could have been derived from the Norse word for "secret, hidden knowledge".

Ron Weasley - See "Weasley, Ronald Bilius"

Rosier, Evan - Death Eater, killed by aurors

From the HP Lexicon: Evan = Greek, 'of noble birth', appropriate for a typical Death Eater with pureblood prejudices.
Rosier is the name of a fallen angel (formerly holding the rank of Dominion in the angelic hierarchy). This demon may appear as either a man or a woman, and is a kind of patron devil of tainted love and seduction, especially noted for assuming a human form of great physical beauty.

Rosmerta, Madam - Owner of The Three Broomsticks

"Rosmerta" is a Gaullish goddess and whose Celtic name means 'Abundant Provider', which is pretty accurate for the hostess of a bar/restaurant. (Thanks to Rita Winston)

Rubeus Hagird - See "Hagrid, Rubeus"

Runespoor - three-headed snake

"Rune" (an incantation of mysterious significance, especially a magic charm) + "spoor" (the track or trail of an animal, especially a wild animal).