I think it was aimed at children, and I read it some time in the 90's.
It involved a man who travelled to the moon, and found a tunnel. At the bottom was a girl whose tongue was replaced by a small snake. She said that the snake reveals a person's true self when it bites them. He let it bite him, but didn't change at all. The girl said it was the first time she'd met someone whose true self matches their outward appearance. I think then he rescues her (back to Earth?)
Some details may be hazy, as I think I was about 5 or 6 when I read it.
The drawing style was (I think) western, and in colour. I think the man was extremely strong and muscular to the extent that it was a plot point. There was also something later on about bats - possibly controlled by the main villan? Apologies as all of these later details are very unclear.