It hurts the creature for a second, but usually not for a long time, as they have little to no nerves in that body part. Of course, that only applies to creatures with detachable limbs. If a creature lost a body part that was supposed to be attached, then it would be in a lot of pain. Especially if it decided to sacrifice its head.
Ha, I remember when I was younger, encountering my first lizard. I didn't want to get bitten, not knowing the innocent little thing I was about to terrorize wouldn't do anything of the sort, so I picked it up by its tail. Needless to say, I screamed when the tail popped off and lie there twitching back and forth on the ground.
Aw.... That's really sad. I never really thought someone could take something as simple as a lizard losing it's tail and make it so deep... But, I suppose sometimes losing "someone" is for the better in the end...?
I really like how this is stylized and a bit different from your other work. Actually, when you posted this on Tumblr, I didn't realize you drew it... It just shows you have a really nice range of styles. I like the simplicity and the message.