Russian-speaking fraudsters of all kinds have a well-known proverb: "Лох - не мамонт, не вымрет" ("A patsy is not a mammoth, it will not die out"), which means that it will always be easy to find a victim for a new fraud.
Thanks to this proverb, the word "mammoth" in the context of various illegal actions is used in the meaning of "future naive victim".
In this case, it's about a future victim of a phishing attack, unprofessionalsomeone not professional enough to defend against them.