The Bible does of course talk about great creatures of the deep. The Hebrew word for these is taniyn, and it is variously translated by the lexicons and dictionaries depending on the context (Gen.1:21; Ex.7:9-12; Deut.32:33; Job 7:12; ; Ps.74:13; 91:13; 148:7; Is.21:7; 51:9; Jer.51:34; Ezek.29:3; 32:2). As you can see from looking at these references (esp. Gen.1:21), rather than being specific the term seems to be somewhat generic, and as such could encompass all manner of giant sea creatures / serpents. Thus the word may comprise only those creatures of which we are aware or it may also include some of which we are presently ignorant (extinct or not). So beyond this, I don't see anything that can be derived specifically from scripture about the existence or non-existence of hypothetical creatures not presently included in the scientific "inventory". You might have a look at the following link as well: "Dragons in the Bible?"