It is actually a noun and based on its nature it is also used as adjective.
Mammoth is actually any species of large, hairy, extinct elephants of the genus Mammuthus. As the Mammoth is usually huge and enormous it is used as an adjective to indicate anything that is extremely large i.e. gigantic in nature.
huge, enormous, mighty, colossal, massive, gigantic, stupendous, king-size, titanic, Brobdingnagian.
1)It was no doubt a mammoth task but we could depend on him as he was a man of alacrity and acumen.
2)From a small industry, he formed a mammoth organization which seems to be spread across the globe.
3)The children are excited about their adventure they are going to have in that mammoth ship.
4)It is a mammoth disappointment for us after the lot of hard work we poured for this project.