Sam Celia has a perfect explanation of what Bowser is:
After much research, I have stumbled upon an answer that is sure to please both parties. Millions of years ago, there existed the largest land turtle that ever lived: Meiolania (my-o-LANE-ee-a). This creature, whose name means "lesser ripper" because it was mistaken for a gigantic Komodo Dragon, was 14 feet long, with a head two feet wide. On its gigantic head it supported two, humongous cow-like horns(just like Bowser) which protected its neck because it couldn't pull its head into its shell. Unlike other tutles, which have tiny, short tails, Meiolania had a thick, long tail with a row of spikes down the middle(also just like Koopa). While only the sides of its shelled carapace were somewhat spiky and it obviously did not have a fiery red mane(though in the actual SMB game, neither did Bowser), I believe Meiolania is a perfect match for the King of Koopas.
