From Longman Dictionary of Contemporary Englishsheepdogsheep‧dog /ˈʃiːpdɒɡ $ -dɒːɡ/ noun [countable] 1 TAHBAa dog that is trained to control sheep2 
DHPa dog of a type that is often used for controlling sheep → Old English sheepdog
DHPa dog of a type that is often used for controlling sheep → Old English sheepdogExamples from the Corpus
sheepdog• He had seen a sheepdog trial in Cumbria: amazing creatures.• Moreover, the most difficult tricks to teach a sheepdog are those most removed from its ancestral hunting skills.• He had turned from a beer-bellied, ranting bear into a well-groomed, grumbling sheepdog.• I was returning along a path and, when passing an isolated croft, a man and his sheepdog emerged.• Surprisingly, relatively few women saw sheepdog work as a useful contribution they could make to the farm.• Lesson: never let an unfriendly sheepdog get behind you. 