Solved. Unlike other pets, you cannot "/tar" a Sneaky Marmot. Although it is nearly invisible, I was able to find it when I walked forward and pointed the zoomed-in camera behind me. Successfully stoned!

I would like to apply a Marked Flawless Battle-Stone to my Sneaky Marmot but am unable to target her. Is there a macro or addon that can apply the stone directly to a pet?

Thanks