Kitesail Larcenist
Creature — Human Pirate
Flying, ward 
When this creature enters, for each player, choose up to one other target artifact or creature that player controls. For as long as this creature remains on the battlefield, the chosen permanents become Treasure artifacts with "
, Sacrifice this artifact: Add one mana of any color" and lose all other abilities.
Illustrated by
The Lost Caverns of Ixalan#364RareEnglishLegality
Rulings
The targets of Kitesail Larcenist's enters-the-battlefield ability will lose any other subtypes and card types they previously had and will be only Treasure artifacts as long as Kitesail Larcenist remains on the battlefield. They will retain any supertypes they had.
If some of the targets, but not all, are illegal as the ability resolves, the illegal targets will be unaffected. Only the remaining legal targets will become Treasures.
If Kitesail Larcenist leaves the battlefield before its enters-the-battlefield ability resolves, that ability will have no effect. The target permanents won't become Treasures at all.