Captain Ahab would be the only one mighty enough to tame the alpha leed. MOBY DICK!

They Spawn Mainly At Herbivore Island, UpVote People Need To *Sea*

Had a knocked out T-Rex on a taming boat and encountered one of these.
This guy puts the dick in Moby Dick.

It will attacks rafts so be carful coming across one on a boat.
Also the Alpha Albino Leedsichthys Will give you raw prime fish me when you scrape meat off of it, and killing it will give you The Captains Hat Skin