Horses were connected to the Celtic goddess Rhiannon, a goddess of hunting.  The Celts showed their respect for horses with huge carvings, which can be seen in hill cliff sides at several locations in Europe.

"Trotting Trio"

This triangular Celtic knot is said to represent the three planes of existence - physical, mental and spiritual. Notice how, although the three points are clearly separate, they are interlocked together, symbolising the connection between the three planes.  Like the triskel, the symbol could also be linked to the trinity of the goddess, or the Christian trinity.