Final answer:
The implausible myth was that Hard Rock had given a guard syphilis by biting him, which is medically unlikely as syphilis is usually sexually transmitted.
Step-by-step explanation:
The myth about Hard Rock that seemed too far-fetched for the inmates to believe involved the claim that he had given a guard syphilis by biting him. This claim stretches credulity as it suggests a scenario that is not medically plausible, given that syphilis is a sexually transmitted infection typically not transmitted by biting.
The Stanford prison experiment, conducted by psychologist Phillip Zimbardo, highlighted the ways in which power dynamics and role-playing in a simulated prison environment could lead to abuses and a loss of objectivity, which could inadvertently foster the proliferation of unbelievable myths like the one about Hard Rock.