Yes this is an interesting western proverb. I had to ask for explanation when I first heard it. This was not something that was taught in my English classroom when I was in school.
So as the horse grows older, the teeth get longer. By looking in the mouth of a horse one can tell how old or young the animal is. In other words, don't scrutinize a gift or be ungrateful; all gifts convey a good will.
I suppose a horse does not have facial wrinkles or nasolabial folds to reveal the age, so the marker rests on teeth. What else can we describe a horse with? I myself am quite intimidated by horses. They are huge and they kick. Their neck muscles are so strong that they can sideswipe me with ease. I have actually fallen off a donkey during one of those National Park excursions. My donkey was having a bad day and bucked me off. I fell off my ass on my ass, pun intended. The fall was enough to break the viewfinder of my camera. Fortunately I was wearing a helmet.
Yet horses are beautiful animals. The musculature on a horse is well-placed and in harmonious proportions to the body, unlike the pumped up looks of body builders of our own species. I want to try to paint them, properly that is.
When I tried to do figure drawing, my instructor loaned me a pristine copy of an illustrated book on human anatomy, muscle anatomy to be exact. Apparently it was seldom handled. Perhaps students did not want to invest the time to study human musculature just so they could paint bodies. That was before the advent of the computer or the internet, unlike these days when all kinds of references are at the fingertip. I grew up with black and white cabled television with a dinner plate size screen. Thermal-paper copy machines were the rage; replacing carbon copy papers on the typewriter. Yes I am a fossil. The idea of studying human muscles was to help with the articulating of the human form. I found out that's how Da Vinci acquired his acumen on human figures. Strangely enough, the muscles of a human body did not intrigue me that much. The only muscle that I was interested in was the sternocleidomastoid when it was attached to the clavicle, especially those of the fairer sex. My friends tell me that is a fetish. I am not so sure about that. That's why when I painted Rusalka I was so intent on accentuating that muscle.