In Greek mythology, Orthrus (Greek: Ὄρθρος , "Orthros") or Orthus (Greek: Ὄρθος , "Orthos") was, according to the mythographer Apollodorus, a two-headed dog who guarded Geryon's cattle and was killed by Heracles. He was the offspring of the monsters Echidna and Typhon, and the brother of Cerberus, who was also a multi-headed guard dog.
Cerberus had more heads than Orthrus.
It says Cerberus was a multi-headed guard dog, and Orthrus was a two-headed dog, but it doesn't say how many heads exactly Cerberus had, perhaps he was also a two-headed dog, in which they would have an equal amount of heads. It's difficult because perhaps the system thinks "multi-headed" is guaranteed to mean more than "two-headed".
( z1 have-quant :ARG1 ( z2 head :part-of ( z3 animal :name ( z4 name :op "Cerberus" ) ) ) :ARG3 ( z5 more ) :ARG4 ( z6 head :part-of ( z7 animal :name ( z8 name :op "Orthrus" ) ) ) )