Icy Mammoth Trunk is part of Mammoth House located on Svargrond . In my opinion you can't obtain the Icy Mammoth Trunk. I have proof that it was possible to get it a few years ago (screenshot is from 2012) probably because of bug. I will try to talk with old owner of this item and ask about more details :).
Edit: I talked with Babini from Secura she was owner of Icy Mammoth Trunk few years ago. She had it on Harmonia.
19:41 Sayens [9]: Did you own Icy Mammoth Trunk?
19:41 Babini [460]: ye it was mine haha
19:42 Sayens [9]: Did you take it from Mammoth House ?
19:42 Babini [460]: yea
19:42 Babini [460]: I could only take 2, then they made them not movable
19:43 Sayens [9]: u lost it I guess?
19:43 Babini [460]: si
19:43 Babini [460]: it was on refugia too
19:43 Babini [460]: gave to my friend when i retired and he lost prem and house
19:54 Sayens [9]: it was possible to put Icy Mammoth Trunk to bp?
19:54 Babini [460]: noh
19:54 Babini [460]: only drag
19:54 Sayens [9]: I heard someone had it in Liberty Bay
19:54 Babini [460]: maybe got it through uhm
19:55 Babini [460]: adventurers guild
19:55 Babini [460]: the tps