The horns of the Hindu and Irish (and other) bulls and native buffalo in mythology and of Bronze age Scandinavian helmets I think at least partly represent the two crescent moon horns just before and just after new moon. This is known in Scottish tradition according to F. Marian McNeill's The Silver Bough Vol. 1). This I relate to the dark night of enlightenment possible before new moon (my five waning crescent highs from 1991 to 1994) and the dark night of despair (dark night of the soul) after new moon (well, that is the two early waxing moon trials I had in early 1995. But perhaps there is a good side to the waxing crescent that I have not seen yet, such as the onset of a new age in waxing crescent and/or the release of me from my low years in waxing crescent. Also while the waning crescent highs are pleasant (except my first one in late stages, i.e. the thorn hill climb) they are intense and the coming of the waxing crescent could signal that you have made it through and wound down to normalcy.
I also found, in a book entitled The Horn and the Sword, by Jack Randolph Conrad, a picture of the earliest known cave painting of a bull chasing a man, from a cave wall at Remegia, in the Spanish Levant. He says the picture is from another book by H.J. Bandi and J. Maringer, entitled Art of the Ice Age. This painting, with the dark bull, I can relate to the story of the Welsh druid Taliesin fleeing the "crack of the cauldron" and the North American aboriginal story of The Raven and the First Men.
The sacred knife of the Sikhs and the sickle in much heraldry and the sword of Kali are also related to the waning crescent horn.
Of course there are other interpretations of the bull which have more relevance for those not affected by such cycles. These interpretations include ones of fertility, animalistic connectedness with nature, and strength. Shamanic poetry and artwork symbols contain multiple meanings.
About the Irish Tain series about Cu Chulainn and others, I speculate that one bull represents the lunar/solar effects on the druid, the other represents the druid battling those cycles, and the number slain or the numbers of days mentioned in the Tain may represent time periods. However it has been at least two years since I read it and I will have to reread it again someday to check.
Other famous horned celtic figures include Cernunnos, who may be represented, or a horned shaman on the Gundestrup cauldron. Also Amergin says that "I am a stag of seven tines" which I believe to mean that he was a seventh child like me. Such a stag, with seven tines on both its large horns, is in the Newfoundland museum.
I was corrected by Asatru who say that Viking warriors did not wear horns, but still believe that some of their shamans did. Certainly Bronze Age Scandinavian pagans wore horned helmets. Some pictures of these (and of horned figures) are in the book by H.R. Ellis Davidson called Scandinavian Mythology, on pages 21, 24, 41, 49, 99 (boar helmet, one wing/horn?), 101, 107.
Some more comments on the bull stuff:
Again this should have some relevance in linking Krishna and other Hindu bull deities, Mithras, Baal, Zeus, Norse, native, celtic, other paths, plus in terms of the new moon mystics, to Christian, Buddhist, Islamic, Baha'i, Jewish and other paths.
I think the cow can represent the full moon for some, like the full moon release of Jesus from psychic death, the full moon victory of the Buddha over Mara against the bodhi tree, Krishna's victory over demons sent by Kama, Taliesin's release from the prison of Arianrhod, the North American native Salmon's ascent from the underworld. The full moon is a sign for the mystic poet to relax, to celebrate, the full dipper of milk tipping over, perhaps for Krishna safe in the arms of his bif cowherd friend(s).