there are a number of Psalms that use a word referring to some creature unknown to the translators that the KJV renders "dragon"
Thou didst divide the sea by thy strength: thou brakest the heads of the dragons in the waters.
(Psalm 74:13)
Thou shalt tread upon the lion and adder: the young lion and the dragon shalt thou trample under feet.
(Psalm 91:13)
Praise the LORD from the earth, ye dragons, and all deeps
(Psalm 148:7)
that last one seems to indicate that whatever creature the psalmist was referring to was simply a natural beast, and not inherently evil. there is a difference between the symbolic language of Revelation where Satan is clearly called a dragon and other verses like these that are referring to some created animal.
looking at the KJV, you have to remember that it's written in an old form of the English language, and a lot of words carried meaning at that time that is largely lost on people today, if they're not familiar with the idioms and connotations that would have been common 400 years ago. i always say, the more Shakespeare you read & understand, the more sense the KJV will make!
not to comment on the usefulness of the KJV versus any other translation. i love it for the beauty of it's phrasing, but i'm not any kind of "kjv only" nut.