Looking at your horses incisors are a good way to determine if they need dentistry.  Sometimes horses are born with a deformity that creates a mal-occlusion problem that will need to be dealt with through out their lifetime.  Many times incisor mal-occlusions are created by uneven wear patterns that can be corrected and maintained with regular dentistry visits.

There are six incisors on the upper jaw (maxilla) and six incisors on the lower jaw (mandible).  They are used by the horse to bite and bring food into the mouth.  An example would be biting grass to bring into the mouth for chewing.   

Foals will have their center incisors at 6 days, the intermediary will be at 6 weeks, and the outside will be at 6 month.  They will loose their deciduous (baby) incisors at intervals of age 2 1/2 for the center, 3 1/2 for the intermediary and 4 - 4 1/2 for the outside incisors.  The permanent teeth are larger in size than the deciduous teeth.     

Since you can see the incisors by lifting the lips, it is easy to determine if they are having issues that need to be addressed by proper dentistry.  Floating the teeth does not resolve incisor issues.

Too many incisors
                            


Too few upper incisors with mal-formations.  This horse doesn't have good lateral movement.
                             

This horse has retained deciduous (baby) teeth behind the permanent teeth that are erupting at age 2 1/2 - 3. 
                            

This horse was born with the Maxilla (upper jaw) longer than the Mandible (lower jaw) creating an overbite. The second picture shows the correction and while you can now see the tongue, it was protruding from below before, it was just covered up by the length of the teeth.  Now the horses upper jaw was actually able to sit back farther than before the correction.  He also has had damage to his lower central incisors.
   \

Now for the opposite and underbite, (Monkey Mouth).  The Mandible is longer than the Maxilla.