The mammoth belongs to a vanished age, so it is read as the past, ancestry and memories frozen in time. A mammoth in a dream can also stand for something huge that seems out of place now.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.