Nagini
In order to survive in his "hideous baby" form, Voldemort needed to drink regular doses of a potion created from unicorn blood and snake venom. He kept a large snake named Nagini around for this purpose. Among his other custodial duties, Wormtail had to milk the venom from the snake at regular intervals. Voldemort could speak with Nagini, of course, and he promised first Wormtail and then Harry to the snake for a meal. (GF1,
"naga" is Sanskrit for "snake"
"In Buddhism and Hinduism, Nagas are a race of semi-divine snakes with great powers..."  (pp. 131-132)

Niffler
XXX
The Niffler is a black fluffy creature which burrows in dirt as if it were water. Nifflers are strongly attracted to anything shiny and they can be very useful for finding treasure. (GF28, FB)

Nogtail
XXX
The Nogtail is a Dark Creature which curses farms. It is a demon that resembles a piglet. The Nogtail slips into the pigsty and suckles an ordinary pig; the longer it goes undetected, the longer the curse that falls upon the farm. The Nogtail is difficult to catch, but if chased off the farm by a pure white dog it will never return. (FB)

Norbert
A Norwegian Ridgeback dragon, hatched from an egg by Hagrid and raised for several weeks in the spring of 1992. Norbert outgrew his surroundings rather quickly and had to be passed along to a group of dragon keepers in Romania. (SS)

Nundu
XXXXX
The Nundu is considered by many to be the most dangerous beast in existence. It is a gigantic leopard, native to East Africa, whose breath carries disease and death. Whole villages have been wiped out by the Nundu. (FB)

Occamy
XXXX
This beautiful creature is native to the Far East and India. It looks like a feathered, winged snake, up to fifteen feet in length. Occamy eggs are pure silver. (FB)

owls
These birds provide postal service for the Wizarding World. Although Muggles know about them, they are obviously magical to some extent since their ability to find the recipient of their letters is extraordinary indeed.

Phoenix
XXXX
Magical bird, the size of a swan, with red and gold plumage. The Phoenix can disappear and reappear at will. Also, it lives through its entire life cycle, dies in a burst of flame, and then is reborn.
Dumbledore has a pet Phoenix named Fawkes from whose tail the feathers came for Harry's wand and Voldemort's - (CS12)

Pixie
XXX
Natives of Cornwall, these mischievous creatures are a bright electric blue in color. They are very rude. (CS, FB)

Plimpy
XXX
The Plimpy is a kind of fish. It is shaped like a ball with two long, rubbery legs and webbed feet. If you happen to spot a Plimpy with its legs tied in a knot, you will know that Merpeople are around. (FB)

Pogrebin
XXX
These annoying little creatures are native to Russia. They love to follow people around, infusing them with a sense of hopelessness until the human collapses, at which point the Pogrebin attempts to devour them. The Pogrebin resembles a grey rock with a small hairy body and it hides by crouching down and pretending to be nothing but a harmless stone. (FB)

Porlock
XX
The Porlock is a horse-guardian found in Dorset, England, and southern Irelan. The Porlock lives to guard horses and will be found with a herd, although it is so distrustful of humans that it is seldom seen at all. The Porlock is a short, shaggy creature which walks on two cloven-hoofed feet. (FB)

Puffskein
XX
Puffskeins are pleasant creatures. They are little furballs with long tongues which make a calming purring noise. Puffskeins are often kept as pets. Ron Weasley used to own one.
(If you immediately thought of "tribbles" from STAR TREK when you read about the "custard-colored furballs" in the Magical Menagerie, you're not alone.)

Quintaped
XXXXX
One of the most dangerous of magical beasts, the Quintaped is found only on the Isle of Drear off the north tip of Scotland. There is a fascinating legend about the origin of the Quintapeds which involves a feud between two wizarding clans and a poorly thought-out magical attack. The reader is urged to read the full account in a reputable source such as Fantastic Beasts and Where to Find Them by Newt Scamander. The Isle of Drear has been made Unplottable to protect the unwary. 

rabbit
A fat white rabbit in the Magical Menagerie transforms itself into top hat and back again. It is unclear if this is a magical creature or if it has simply been enchanted to preform this bit of magic (PA4).
Lavender Brown had a pet rabbit, just a baby, which was killed by a fox. It's name was Binky. (PA8)

Ramora
 

rat
Scabbers was Ron's pet rat for many years. That's not nearly ALL he was, of course... (SS, CS, PA)
In the Magical Menagerie was a cage full of black rats which were busy jumping rope with their tails (PA4).

raven
Ravens were for sale on Diagon Alley (PA4)

Red Cap
XXX
Small, goblin-like creatures, Red Caps love bloodshed; they bludgeon those lost in dungeons or battlefields (PA8). 

According to A Field Guide To The Little People (Nancy Arrowsmith with George Moorse, New York: Hill and Wang, 1977):

Some of the most bloodthirsty Scottish elves are the Lowland castle spirits. They are called Red Caps...and by some accounts are said to live only in castles with a history of violence. Others say they live in all Lowland peel-towers [fortified houses or towers]. It may well be that both accounts are true...The Red Caps' main occupation is colouring their red caps, which they dye with human blood. They throw boulders on to travellers from their towers, then catch the blood in their caps. As soon as the blood dries and the colour fades, the Red Caps look for new victims. They also foretell disasters by making a loud noise like the beating of flax. The only things that discourage them are crosses, cross-handled swords, and words from the Bible. (p. 47)

Re'em
 

Runespoor
 

Salamander
XXX
The Salamander is a fire-dwelling lizard, small and orange, which scampers along the burning logs and among the coals.
George and Fred feed fireworks to one just to see what happens (CS8). Hagrid provides bonfire full of them for Care of Magical Creatures class (PA12)

Scabbers (actually an Animagus)
     introduced (SS6)
     bites Goyle (SS6) ("His finest hour!")
     really Peter Pettigrew (PA

Sea Serpent
XXX
 

Shrake
XXX
 

snails
Poisonous orange snails were for sale in the Magical Menagerie (PA4).
Could be Streelers (FB)

snakes
and snake-like creatures

• Nagini - (GF1)
• boa constrictor - Harry accidentally released from the zoo, frightening Dudley half to death and resulting in a severe punishment from Uncle Vernon (SS2)
• Basilisk
• dragon
• Ashwinder
• Occamy
• Runespoor
• Sea Serpent
• A snake comes out of Malfoy's wand from the "Serpensortia" spell (CS

Snidget
XXXX (the high rating is because of the severe penalties attached to capturing or owning one)
The Snidget is a small, spherical bird which can fly with amazing agility, changing speed and direction almost instantaneously. It's golden feathers and red, jewel-like eyes are so prized that at one time the Snidget was hunted almost to extinction. The fact that a Snidget became such an integral part of the game of Quidditch (and usually died when it was caught) didn't help matters either. The Snidget became protected and there are now severe penalties for harming or even capturing one. Snidget reserves have been set up worldwide, and a magical device, the Golden Snitch, has replaced the live bird in Quidditch. (FB) 
 

sphinx
XXXX
Native to Egypt, the Sphinx is a lion-like creature with the head of a human female. The Sphinx is used by Witches and Wizards to guard treasure, and can be ferocius when 
(GF31)
in the Triwizard maze
required an answer to its riddle
 

spiders
and spider-like creatures

• Aragog and Mosag (CS15)
• Acromantula
• Ron Weasley's greatest fear is spiders, ever since his brothers turned his teddy bear into a giant spider when he was little (CS7, PA
• cupboard under the stairs was full of them (SS2)
• Moody used three spiders to demonstrate the Unforgivable Curses (GF
• giant tarantula
• Harry encountered a gigantic spider in the maze of the Third Task. He and Cedric defeated it together. (GF

squid, giant
A giant squid lives in the lake near Hogwarts castle. Although students are afraid of it, it seems to be more or less friendly. (PA22)

•      Fred and George tickle its tentacles (SS
•      recues Dennis Creevey (GF12)
•      eats toast (GF18)
•      lazily crosses the lake