There are so many parts of the horses head that give us clues to their temperaments.
The chin and lips are some of these. They are unique from things like eyes and ears in that they are muscle.
The shape of eyes, ears, and profile are fairly well set. The horses are born with them and they don’t change. Chins and lips change and grow with the horse as the horse matures and develops their own unique personality. Which brings us to a very important question.
Does the shape of the chin and lips, the muzzle, show what the temperament will be or does the temperament cause the shape of the muzzle?
Frown lines don’t cause a person to spend their time frowning. Frowning causes frown lines to develop. Or, to the contrary, laugh lines. The way a person thinks, acts, and holds themselves is the cause of the way the lines of the face develop.
If a horse is curious and wants to explore with their mouth does this develop the muscles in the upper lip making it shapely and curvaceous? If a horse carries tension in tightly drawn lips does this build the muscling of the lips and chin adding lines, ridges, and layers for a complicated chin?
Some things, certain shapes and characteristics, are inherent and unchanging. A chin that comes to a point that gets hard with the lip flattening and extending when the horse is upset, is a born trait that accompanies several other traits. The sensitive skin that tends to wrinkle more than normal, that accompanies this chin type is far deeper than muscle development.
Looking at the way the muzzle develops over time can give us a lot of information. How much is cause and how much effect? That is hard to say.