The Welsh had a pantheon of gods and goddesses before the coming of the Romans. With the defeat of the druids and the extermination of their sites on Anglesey, the druid religion in Wales went into decline–and perhaps that is the reason there are relatively few Welsh gods and goddesses compared to the Irish, whose religion flourished during the Dark Ages and also developed a unique form of Christianity alongside it.
Within the belief system, faeries, or Tylwyth Teg, the modern designation, had a role, divisible into five classes: the Ellyllon, or elves, the Coblynau, or mine fairies, the Bwbachod, or household fairies, the Gwragedd Annwn, or fairies of the lakes and streams; and the Gwyllion, or mountain fairies. http://www.sacred-texts.com/neu/celt/wfl/wfl02.htm
Ellyllon: “The Ellyllon are the pigmy elves who haunt the groves and valleys, and correspond pretty closely with the English elves. The English name was probably derived from the Welsh el, a spirit, elf, an element; there is a whole brood of words of this class in the Welsh language, expressing every variety of flowing, gliding, spirituality, devilry, angelhood, and goblinism. Ellyllon (the plural of ellyll), is also doubtless allied with the Hebrew Elilim, having with it an identity both of origin and meaning. [Pughe's 'Welsh Dictionary.' (Denbigh, 1866)] http://www.sacred-texts.com/neu/celt/wfl/wfl02.htm
Coblynau: “The Welsh version of the Cornish Knockers, these mine spirits were relatively good humoured, and helped the miners by knocking in places with rich lodes of mineral, or metal. The Coblynau dressed in miners’ attire, and stood at around 18 inches in height. Belief in these mine spirits was once widespread especially in Celtic areas which were heavily mined, for example Wales and Cornwall.” http://www.mysteriousbritain.co.uk/wales/folklore/the-coblynau.html
Bwbachod: “The Bwbachod or otherwise known as the Bwca or Bwbach is a Welsh household spirit. The Bwbachod performs tasks when appreciated but becomes mischievous and destructive when offended. The Bwbachod detest people that don’t drink alcohol . . .” The modern update is that this creatures also hates church and ministers. http://www.mythcreatures.co.uk/celtic/bwbachod.asp
Gwragedd Annwn: “The Gwragedd Annwn are Welsh water faeries who live in towns and villages beneath lakes. Often using glamour to disguise these dwellings, the most famous example of which was the Lady of the Lake, whose palace was disguised with a magical lake. Gwragedd Annwn as kithain do not live in underwater communities like those of their faerie cousins; the relative lack of glamour makes this impractical, and the encroachment of humanity, and the pollution it brings with it, on lakes and bodies of water makes the construction of underwater cities impossible. Now the Gwragedd Annwn have to cling to what sparing freshwater they can safely haunt.
They have an innate aptitiude for all things medicinal. There are reports of secret gardens hidden on islands in the middle of lakes, which are impossible to find except by a special entrance which is only opened on New Year’s Day. Everything in these gardens is sacred and removal of even the most trivial of items, such as a flower, leads to the permanent closure of the garden, except to the Gwragedd Annwn. Now, these precious few secret gardens serve as moderately powerful freeholds, but those which still exist are difficult in the extreme to enter, having long been left by their faerie denizens.
It is said of Gwragedd Annwn who take mortals for partners that if they should be struck three times causelessly, they must leave their partner, never to be seen by them again, and to Gwragedd Annwn, this is as strong as the most powerful oath. Gwragedd Annwn have contrary reactions to most collective emotions; they might cry and lament at a wedding, or laugh and sing at a funeral. These unusual feelings must be learned to be dealt with if the kithain is to get along in ordinary society.” http://www.angelfire.com/ca4/dataweaver/play/changeling/gwargeddannwn.html
Gwyllion: “The Gwyllion is a mythological creature from Wales. Even though these elfish creatures are mostly harmless you should always invite them into your house and treat them well, because if you don’t, it may result in destruction. The female faerie is very hideous and its only job is to cause travelers to become lost. Many times they just bother you or possibly frighten you by sitting on either side of a mountain path and following the traveler with their eyes. These ladies usually like on mountain trails, but if the weather becomes bad they resort to going to the valley. If you do happen to be threatened by a Gwyllion just take out a knife and point it directly at her. It is strongly recommend to have a knife handy if one plans on hiking in the night time, for this is there prime time for terror. Just beware next time you plan on going on any trails.” http://www.pantheon.org/articles/g/gwyllion.html