There is no description on King Daggerfangs himself, but as a "Toothless Daydream" he would look similarly to a Common or Garden Dragon, except be very large in size and have a wart on the end of his nose.
Fishlegs makes up the Toothless Daydream species to apply to Toothless to fool Stoick the Vast. He also claims Toothless is of royal blood, being the offspring of King Daggerfangs.
|“||And it's not just your average Toothless Daydream either, ... This particular dragon is of ROYAL BLOOD. ... Your son has only gone and burgled the offspring of King Daggerfangs himself, the reptilian ruler of Wild Dragon Cliff.||”|