In Greek mythology, which two-headed dog was the brother of Cerberus?

Orthus is the brother of Cerberus. Sadly Orthus only had two heads which probably caused him problems with his relationship with Cerberus. Rather than guard the underworld, Orthus guarded the giant Geryon's cattle. Geryon was the grandson of Medusa...not sure how she had kids to give her grandchildren... If having Medusa as a grandmother is not strange enough, how about having Pegasus as an uncle? I can't make this up! In the end, Orthus died at the hands of Heracles so the hero could complete his tenth labor.

https://en.wikipedia.org/wiki/Orthrus
https://en.wikipedia.org/wiki/Geryon