Canine teeth in horses are located in the bars of their mouth (space between the incisors and cheek teeth). They are spade shaped with thin sharp edges for their points. Canine teeth erupt when horses are 4-6 years old and are usually very painful during eruption. All male horses have 4 canines (2 on top, 2 on bottom) but only about 25% of females have some rudimentary form of canine teeth.
Canine teeth serve no purpose other than as a fighting weapon. They do not aid in chewing at all. Most are large and sharp enough to do damage to the tongue and inside of the lips. Many horses I see have ulcers and scars caused by their canines.
Shortening and smoothing the edges of the canine teeth is an important part of the dental procedure. This helps prevent horses from traumatizing their tongue, especially when the bit is in their mouth. Shortening large canine teeth also allows for easier bit placement and removal.
David Warren, D.V.M.