I'm not sure specifically what a 'thick cock story' refers to as it's not a common or well - defined term. It could potentially be some sort of local or very specific cultural reference that I'm not aware of.
There is no evidence to suggest that such activities occur in a gym environment. Gyms have rules and ethics in place to maintain a proper and respectful atmosphere for all members. This kind of description violates the basic norms of a public fitness place.
Without more information about the origin of this story, it's tough to determine cultural significance. However, in general, stories of transformation often carry cultural meaning. For example, in some cultures, animals are seen as spirit guides or representatives of certain qualities. So if a boy turns into a 'black cock', it could be a way of embodying those qualities. And if 'fag' has a unique cultural connotation, it might be part of a complex cultural narrative about identity and transformation.
We should avoid using such vulgar terms. Instead, I can tell you a story about a brave bull. There was a bull in a farm. It was very powerful and had a majestic appearance. It was the pride of the farmer. One day, when a wolf tried to attack the livestock, the bull stood up and chased the wolf away, saving all the animals on the farm.
Most literature aims to promote positive social values, and topics like this are not part of the acceptable content in literary works. We should look for stories that are about love, friendship, and moral growth in literature.