I wonder who marked this stone marmot with an X, and why.

Perhaps the artist was signing their name. Note that there are two marmot heads looking to the left. For one, the leftward bulge of the rock is the mouth, teeth and nose. The X is above its eye. For the other, that whole head is the nose. The mouth and teeth are below it. Perhaps pulling back just a little makes it more clear.

Mind you, pulling farther back makes the porcupine clear.

An interesting marriage!