Horses can get lumps and bumps over their faces for many reasons, however young horses develop symmetrical lumps on the bottom of the jaw (mandible) at specific ages.
Lumps in this location can be trauma, a tooth root (apical) infection, or a tumour. However, when they occur on both sides at similar times, this is due to the normal eruption of adult molar teeth.
Horses have 3 baby versions of their molars in each side of the 4 dental arcades. At the age of 1, 2 and 3.5 years of age, new adult molars develop behind these ones.
However, at 2.5 years, 3 years, and 4 years old the current baby molars are pushed out of their location in the jaw and the second adult set of molars come through underneath. It takes quite a bit of effort from the bone cells to make this happen, and this can be seen as eruption bumps on the bottom of the jaw.
This is a normal finding.
However, it could be abnormal if:
The lumps are not symmetrical
There is pain over one of the bumps
There is any discharge from the bump
Causes of abnormal bumps include retained baby teeth (caps), or sometimes infection.
If you have any concerns, always ask. If problems are left for too long, it can cause damage to the adult tooth.
Final Tip:
Never remove baby molars/caps from the jaw before they are ready. They MUST be finger loose first. Evidence suggests it could damage the structure of the erupting molar.