From the google dictionary:
zoomorphic: “having or representing animal forms or gods of animal form”
Zoomorphic designs can be found in Pictish rock carvings, and their presence in Celtic art predates Christianity. Both Picts and Celts carved animal forms with stylized swirls incredibly similar to those seen on Viking stones, which suggests they probably all originated from the early European culture that predates the division of the Celtic and Germanic languages. You can view examples here: http://www.pinterest.com/monarose9212/european-petroglyphs/
We know little about the Picts, but I keep running into threads of information on early Pre-Christian Celtic and early Germanic religions explaining how people “worshiped” local rivers, mountains, forces of nature, etc. The time frame corresponds pretty well to the earliest rock carving examples. We do know that animal forms were often “totems” of European tribes and clans, (which later lead to heraldic animals), and may have also represented different forces or spirits or the elements in a similar way to Coyote, Raven, (tricksters), Thunderbird, (Lighting and thunder) etc in Native American mythology.
Carvings of horned human-figures exist in both Celtic and Germanic art, and correspond of course to the spirit of (or the human connection to) the woods/nature/the wild/the wild hunt. The concept of spirits of trees, and lakes, earth and rock, continued on in verbal mythology as the notion of dryads, moss maidens, nayads, goblins, trolls, etc, but they are further evidence of early European animism–albeit anthropomorphisized.
Animism probably roughly corresponds to stone age, and early bronze age. But as people transitioned to iron age (and agriculture), it seems that the idea of animistic forces transitioned into polytheistic gods–gods that were less connected with the land and nature directly–perhaps because people would not want to think about the spirits of living nature when they were so busy domesticating nature? It was much easier to separate the idea of gods from the land). But many of the animals that are associated with the various gods still seem to correspond with the animals seen in earlier imagery. Of course, some of the earlier gods still are more connected with their animistic origins. Thor is one of the oldest Germanic gods, and of course corresponds to thunder and lightning in an anthropomorphisized form.
(I just edited all this, I typed it quickly before and it needed to be made quite a bit clearer)