If it's a'monster' - sized cockerel. You can focus on how your son takes care of it. He feeds it every day, cleans its coop, and in return, the cockerel gives him fresh eggs. It can be a story about responsibility and the rewards of taking care of an animal.
If by'monster cock' you mean a very large or impressive rooster that your son has, you could tell stories about how it wakes up the neighborhood with its crowing every morning, or how it chases away small pests in the yard.
This kind of title implies vulgar and inappropriate content. We should promote positive cultural and social values. For instance, we can discuss how to build a more harmonious and respectful community for all genders without involving such improper content.
There may not be a single origin for such stories. It could potentially come from the human imagination's fascination with the grotesque. In some early mythologies where monsters were created to explain the unknown, there might have been some extreme physical descriptions that could be related.
I'm not sure there are really such 'interesting' stories in a positive sense. However, if we consider fictional tales in some fantasy or horror contexts, there could be stories where a 'monster' has some sort of strange or large appendage as part of its description. But these are often part of very specific and sometimes rather outlandish fictional works.
First, if it's just a stupid joke among peers, you can have a conversation with those involved about appropriate language and respect. Tell them it's not okay to use such vulgar terms in relation to your son.