Sorry TMI, but in occasion (startled, excited, being groomed) my dog's anal glands sometimes squirt out thin black liquid that stinks to high heaven like rotten fish. Last time the vet squeezed them out and said it was common in puppies for them not to empty efficiently, but she is older now and I don't know what's normal or not.
She poos normally. And is fit and healthy. Aged 1. She doesn't lick the area or rub her bum on the floor.
I've been to the vet so many times lately with other stuff (joints and eyes) that I don't want to bother them with this unless I really need to.