What are the marks called on the incisors of a horse?

Answer 1

Galvayne's groove runs down the corner incisors of horse later in the animal's life and can, to a degree, indicate age. Horses are initially aged based on eruption of adult incisors, then based on the
loss of cups on the grinding surface of the incisors and finally based on the cross sectional shape of the teeth, the angle at which the front teeth meet (side view), the gum line and Galvayne's groove.

Horses with abnormal wear for their age (often referred to as sand mouth) cannot be correctly aged using "cups" and the need to use the gum line and angle of the front teeth become more critical.

Horse's that have worn their teeth to the point where all the cups are gone from the surface of the
incisors are called smooth mouthed. (In some locations the term smooth mouthed is used to describe a horse of any age that has no cups...which is a simple way of saying "your guess is your own business and good luck to you!"

Galvayne's groove begins to show at the top of the outside incisor at around 12 years and etypically halfway down the tooth at age 14-15. At 16 the groove usually runs down the entire tooth. After 16
the groove begins to disappear from the gum line growing out completely in the 18-20 year range.

Aging a horse using its teeth is not an exact science since tooth wear can be slowed or increased based on diet and exposure to course ground/sand.