You can substitute this card for any 1 Fusion Material Monster, but the other Fusion Material Monster(s) must be correct. You can discard this card to the Graveyard: Add 1 "Polymerization" from your Deck to your hand.
There is no other card that does what King of the Swamp does. (Be both a Polymerization and a Fusion sub.)
As just general Fusion support... I doubt we'll get anything that makes King of the Swamp obsolete any time soon. Unless they release Keeper of Dragonic Magic, but King's versatile enough that even that wouldn't likely make King of the Swamp not worth using.
Anonymous No. 2
Was this card erratad?
Because the text in the above picture does not contain the words "... whose name is specifically listed", while the additional text is present in the game version of the card and on sites like yugiohwiki.
It might seem like a small difference, but it's really not. It means that it can't replace a generic fusion material, only specific monster material listed on the fusion monster.