Cult associations of inanimate nature
Equally widespread and, in certain circumstances, even more tenacious was the cult of sacred trees. The druids association with the oak has not been proven beyond reasonable doubt, but the tree-cult in general is well attested by Gaulish dedications and nomenclature. It is in Irish tradition, however, that it is expressed most clearly and consistently, and there is here even a special term for the sacred tree, bile, which is no doubt the same word as occurs in the Gaulish Biliomagus, The Plain of the Sacred Tree(?). A number of these great trees are mentioned repeatedly in the literature - the Tree of Tortu (an ash), the Oak of Mughna, the Yew of Ross, the Bough of Dathí (an ash), the Ash of Uisnech, &c. - but evidently each tribe, or confederation of tribes, had its own sacred tree which stood on the site where the kings of the tribe were duly inaugurated. No doubt, like the universal World Tree, or axis mundi, the tribal tree was supposed in theory to stand at the centre of the tribal territory and to embody its security and integrity. Not infrequently one reads in the Irish Annals of a raiding force invading hostile territory and felling a sacred tree, and quite evidently this was conceived as a dramatic gesture designed to shame and demoralise the people for whom it was both talisman and crann bethadh, tree of life.
Cult associations of animate nature
The climax and finale of the saga Táin Bó Cuailnge is the conflict of the two great bulls, the Finnbhennach, Whitehorned, and the Donn, Brown, of Cuailnge. When the outcome seemed in the balance, Cormac son of the king of Ulster struck three blows on the brown bull and upbraided him for his weakness. The Donn gave heed, for he had human understanding, and attacked with fresh vigour until finally he brought down his adversary. Then he careered across Ireland in his battle rage, scattering fragments of the mangled Finnbhennach from his horns, thereby occasioning the creation of a series of well known historical placenames. When he reached his home in Cuailnge his great heart broke like a nut in his breast, and with his death the saga ends.
In this encounter of the two bulls we seem to have the original nucleus of myth around which the extant narrative of Táin Bó Cuailnge has been assembled. These animals are not of this world: they reached their present state, we are told, only after a prolonged series of metamorphoses during which they assumed the form of ravens, stags, champions, water beasts, demons, and water worms, and in the beginning they were the swineherds of two of the lords of the otherworld. Here the shapeshifting which is such a commonplace of Celtic tradition serves to link the anthropomorphic and zoomorphic aspects of the deity, so that the text becomes almost like a gloss on much of the iconography. For these are divine swineherds, avatars of the herdsmangod who crops up so very frequently in Celtic literature and who has an analogue in the Vedic Pushan, and several in Greek. Thus by one of the myriad involutions which make Celtic mythology both interesting and intractable we find ourselves once more close to Cernunnos, lord of the animals.
The Brown Bull of Cuailnge can scarcely be dissociated from the Tarvos Trigaranus, the Three-horned bull, whose images are found both in Gaul and in Britain . And since names of persons, places and populations commonly echo those of deities, one can safely postulate a conceptual relationship between the Donn of Cuailnge and a number of widely attested names which seem to imply familiarity with the notion of a bull-deity. Particularly interesting is the Gaulish name Donnotaurus, Brown, or Kingly, Bull. It is evident, then, that the divine bull figured prominently in the mythology of all the Celtic peoples, and Táin Bó Cuailnge, greatest of the Irish sagas, is therefore a fitting as well as a lasting monument to his remarkable prestige.
But the bull is but one of a number of animals and birds which had special cult associations for the Celts. Cernunnos was related to the stag, the ram-headed serpent, the bull, and also, in a less immediate sense, to the whole of the animal world. In addition, the iconographic record comprises boars, horses, dogs, bears, &c., as well as fish and various types of birds, all of them connected more or less closely with certain deities. Not surprisingly this diversity is reproduced in the insular tradition, and in such complexity and abundance as to defy any neat classification. Of all these supernatural creatures the boar is probably the most notable. Frequently he is the fierce, destructive quarry who leads his hunters to the otherworld, and generally we are told that he was anthropomorphic until transformed. His mythological importance is not unconnected with the fact -abundantly attested in archaeology and literature -that the Celts esteemed pork as the choicest of foods both in this world and in the next, and in Irish legend the undiminishing otherworld food is sometimes represented in the form of a pig which, though killed ind cooked each evening, is alive and whole the next morning. The Gaulish Mercury occurs with the epithet Moccus (Welsh moch, pigs) and obviously this refers to a native deity concerned  in some capacity with pigs, as hunter perhaps or as divine swineherd.
Supernatural horses are frequent. Sometimes, particularly in folktales, they carry off mortal men to the otherworld of the dead, but generally their role was a sympathetic one: it may be recalled that horse-racing was a feature of the great seasonal assemblies and one of the pleasures especially associated with the otherworld. Epona, The Divine Horse or The Horse Goddess, was one of the more important Gaulish deities, winning especial favour among the cavalry of the Roman army. She may have an Irish analogue in Edaín Echraidhe (echraidhe, horse-riding), and some scholars have seen an equivalent of her in the Welsh Rhiannon. There was also Dea Artio (as well as a Mercury Artaios), obviously connected with the bear (Irish art, bear), and a Dea Arduinna who is shown seated on a wild bore. With this type one may compare the Irish goddess Flidhais who ruled over the beasts of the forests and whose cattle were the wild deer.
Magic or divine birds are equally numerous. There are deities who assum bird form occasionally - one of the characteristic Celtic motifs is of swans linked by a silver chain, the symbol of divine beings metamorphosed – and there are those, the war-goddesses, who do so constantly; one of the names given to them is Badhbh, Raven, Hoodedcrow. There are also the wondrous birds who figure in almost all accounts of the Happy Otherworld and who lull men to sleep with the soothing sweetness of their music; these are assigned to Rhiannon in the Mabinogi and probably correspond to the birds who accompany a goddess on a number of sculptures.
It has been suggested that the wealth of animal and bird imagery in Celtic mythology, and particularly the intermingling of animal and human forms in the characterisation of the deities, points to the coexistence of inferior (or primitive) and evolved forms of Celtic divinity. But such a view rests upon the assumption that, at least on the higher level of belief, there was a gradual evolution from a zoomorphic to an anthropomorphic conception of divinity, and for this there is no real evidence. What is clear, however, is that the Celtic idea of the otherworld, as this is realised in the literature, allowed remarkable imaginative fluidity with the natural and supernatural seeming continually to merge and commingle in an almost free variation, and it is perhaps in this light that one should view the regular and easy interchange of zoomorphic and anthropomorphic images.