Animals and their Religious Attributes
horned god:  virility, wealth, prosperity
ram-headed serpent:  war god, healing
ram: war, warriors, fertility
raven:  prophecy, war, fertility
generic birds:  mother goddesses, messengers
horse:  female fertility, maternity
swan:  sun gods
aquatic birds:  sacred hot springs and other
religious water sites
goose:  ill-omen
eagle:  sun gods, kingship
crane:  transformed humans; mean women
cow:  magic
boar:  warfare, magic
dog, wolf:  companion
bear:  protection, admirable traits
hare:  hunting, success,
fish:  knowledge from the "other side"

Common combinations of animals with
religious significance:
ram-horned snake
triple-horned bull
stag-horned man

Types of wild animals with religious significance:
bears
boars
cranes
crows
ducks
eagles
geese
hares
ravens
snakes
stags
swans
toads/frogs

Types of domestic animals with religious
significance:
bulls
dogs
horses

Horny Little Devil
Both the stag and the bull play strong rolls in pre-Celtic and Celtic imagery, so it is natural to see a major male god depicted with horns. Interestingly enough sites in Hertfordshire, Welwyn and Barton are among the ones who have located artifacts which depict horned animals with knobs or balls at the tips of them.  
The horns are integrated even in typically horn-less animals;  for example a snake with ram's horns usually accompanies the stag god One of the names for this god is Cernunnos.

Swearing on the Horns
The photo is from Highgate in 1906. At one time there were about 20 public houses (bars) in this part of London which made strangers hold a pair of antler horns and swear to basically have fun.  Although this custom has largely died out, a pub called The Wrestlers still performs the ceremony twice a year.  There are other events around London which link to celebrating the Stag-horned god.

The Horn Fair, London
The current one is a revival of the one which continued until authorities stopped it in 1876.  Although gaining popularity, the original had people frolicking in a frenzy wearing play antlers. To gain additional perspective on how far back this tradition went, there is a place in nearby Greenwich names Herne Hill and where St Paul's Cathedral now stands was a temple dedicated to Diana (Roman Goddess of the Hunt).