I learned this treasure in grad school. I was a graduate assistant for a String Techniques class at the time. The lady who was responsible for teaching the class was a talented musician around my age, and we quickly became friends. The students were studying to become teachers themselves, in some type of K-12 school setting in the future. Most of them had a background in band, not orchestra. They were in our class to learn the ins and outs of working with a string section. One day in class, one of the undergraduate students raised their hand and asked, "Why is the frog called the frog?" I chuckled. It was a question we all wondered, even as experienced string players. But I was surprised when my friend actually had a response instead of a shrug! One of her hobbies was caring for and riding horses. She knew her horse anatomy. She also knew her world history. And she had an educated theory as to an answer to that question. The violin, she explained, dated back hundreds of years, and even nomadic cultures had these instruments. These nomads raised horses on the plains. It's common knowledge that the bow hairs for a violin, viola, cello, or string bass, are made from real horsehair. Well, she said, why would it be a stretch to imagine that these nomads referred to other elements of their musical instruments based on horses, something they knew and understood?
She then told us that the bottom of a horse's hoof is called the "frog." Interesting! As we violinists know, the frog of the bow makes up the base or bottom of the bow (where the hand holds it). The correlation is clear! The bottom of the hoof and the bottom of the bow, both responsible for movement and grounding, in their respective ways, were called by the same name! I felt like a light bulb had just turned on. This experience occurred a while ago. More modern blogs (2013, 2020, etc.) on violin-making also corroborate this fascinating theory as a legitimate one. Click here for an example. (In addition, that particular article explains that the frog of the bow used to be called the "heel," a similar comparison . . . . )