Okay, so a much of this comes from the middle ages if not later.  The fact is,
there may be core content in these mythological ideas which stem from an
earlier time.  Besides, I just think this type of information is fun!
Creature
Power/Story
Banshee
her appearance foretells death
Changeling
a nasty little magical creature left in the stead
of a kidnapped infant
Leprechaun
less than 2 feet tall, known not only for their
pot o' gold but also for being shoemakers.  If
you stare and one, and not break your gaze, it
must lead you to its treasure.  It also likes its
alcohol.
Pooka
a shape shifter, it usually chooses the guise of
a large animal.  Some stories include the
creature's red eyes. (A much kinder Pooka
appears in the movie
Harvey)
Merrow-Folk
Mer-people who can live and intermarry
among humans, but always go back to the sea.
The Dullahan
a headless rider on a black carriage lead by
black, headless horses.  An omen of death.
Roanes/Selkies
"seal-people" who, like the Merrow-folk, would
join humanity for a while and then return to the
water.  (A must see movie on this is
The
Secret of Roan Inish
)
Lady of the Lake
of Llyn y Fan
on the first Sunday in August, in this lake in
the Village of Myddfai in Wales, the lake is
supposed to look like it's boiling from the
return of the Lady and her Oxen
Bogies/Bwganod
shape-shifters who like to bother humans
Peists
giant eel-like lake dwellers, like the Loch Ness
monster
Barguest/Church
Grim
a red-eyed dog who foretold death
Boggart
a nasty little creature who loves to torment
people when upset or neglected
Brownie
a general term for magical creature/fairy
Dobby
a hobgoblin in certain parts of the England.  
Offerings to fairies were placed on Dobby
Stones  (anyone else thinking
Harry Potter?)
Fetch
a creature who resembles the person whose
death they are foretelling
Gabriel Hounds
ghost hounds who hunt in packs
Gindylow
a Yorkshire water creature who drags people
under and drowns them
Grant
a shapeshifter goblin whose eyes flash
Gwyllion
a Welsh faery, female, who lures travellers off
their path.  These are very spiteful creatures,
so if you can't stay on their good side - then
get some iron, which keeps them at bay.
Hobgoblin
a kindly spirit  - unless neglected
Knocker
a mine spirit who knocks to indicate a good
spot
Knucker
English swamp dragon
Nixies
water sprites
Oakmen
wood spirits, especially in Northern England
Shag Foal
a spirit in donkey form with flame-red eyes
Sidhe
these are the faerie folk, probably vestiges of
local pre-Christian beliefs.  In Aviemore,
Scotland lore states if you come across a local
faery it will take you to its home in the hills for
a feast, complete with music and dancing.  If
you partake of the food or drink, you will
forever remain there celebrating and never
growing older.
Silkie
another term for a ghost
Thrummy Cap
a cellar ghost
Tylwyth Teg
small folk who steal things; they can live in the
ferns on a mountain in summer, in the heather
in winter and like to gather in the mist; they are
infamous pickpockets who replace the victims
money with fairy money
Men an Tol - this site in Cornwall was originally 2 upright stones
in a triangle with a doughnut shaped stone in the center (see left).  
It's the circular stone which draws the superstition:
  • if you take a naked child through the hole 3 times and drag
    them along the grass towards the East 3 times, you can cure
    rickets and some other illnesses
  • if you have arthritis or back problems, it was recommended to
    crawl through the hole 9 times away from the sun.
  • you can have questions answered by laying 2 brass pins
    cross-wise on the round stone.  They will move to answer
    your questions.
Hungry Grass
If you walked over an
enchanted section of land, you
could suffer from immediate
starvation and die shortly
thereafter without assistance
Puck/Will O'the Wisp/Water
Sheerie/Jacky Lantern
Whether over bog or solid land, it
was a light which would lead
travellers astray.  No matter how
close you came, you could never
catch up.
Trees/Plants it is bad luck to
chop down due to magical
protection:
Ash
Hawthorn
Oak
Rowan