Dogs with colitis will have mucus in their stools. Collitis is irritation of the intestine and can be caused by a range of things from a change in diet or consumption of a large amount of indigestible fibre like grass, to parasites like whipworm or protozoan parasites. There are also some more serious ailments that case this so if it is a permanent rather than intermitant problem then I would recommend a visit to your vet.