The jackal mastiff was a type of Klingon animal used as a form of guard dog.
The Klingons used jackal mastiffs to guard the prisoners on the penal colony of Rura Penthe. (Star Trek VI: The Undiscovered Country)
The jackal mastiff, which was identified by name in the script and end credits, was created by Greg Cannom. A reference to a "Klingon jackal" in "Tears of the Prophets" may have been referring to this animal.