The reason for the recess in the surface of frogged bricks being called a frog is not known.
A:One theory is that as the block used to press this indentation is also called a frog, possibly based on the appearance of this block especially if some clay adheres to it, and the name was transferred to the recess created in the brick by the frog.
A:Another theory is that the similarity between the recess in a brick to the recess under a horse's hoof is the origin of the name "Frog".
The compacted mud that drops out of a horse's hoof can look like a crouching frog or toad.
