An erect penis that is beyond large, beyond thick, beyond long, beyond extra-long, and which stuns the (lucky?) looker-on. Unusual shapes figure into the formula too, such as a very long thick shaft with a very small glans (head). The judgment as to whether a penis is a freak of nature
is somewhat subjective, although there's a consensus that a true freak of nature doesn't look quite normal, no matter how impressive it may be.
"I saw an Al Parker movie once. Man, was he a freak of nature!"
"Yeah, I could never reconcile that size dick with that size body. But even though Parker was huge, he wasn't freak-of-nature huge by size alone. He was rather small in body, and it's the discrepancy between Al and Al's erection that made him look rather, um, unworldly."