The phrase "pretty-tranny" is a combination of two words: "pretty" and "tranny." In the context of adult content, "tranny" is often used as a slang term for transsexual or transgender individuals. So, "pretty-tranny" would mean an attractive transsexual or transgender person, potentially suggesting a video featuring such individual or content involving a sexual encounter with a transsexual or transgender person.
As a reminder, this description is meant for adult audiences experienced with porn videos and the context of adult entertainment.