Smith's Bible Dictionary
The translators of the Authorized Version, apparently following the Vulgate, have rendered by the same word "dragon" the two Hebrew words tan and tannin , which appear to be quite distinct in meaning.
- The former is used, always in the plural, in (Job 30:29 ; Psalm 44:19 ; Isaiah 34:13 ; 43:20 ; Jeremiah 9:11 ) It is always applied to some creatures inhabiting the desert, and we should conclude from this that it refers rather to some wild beast than to a serpent. The syriac renders it by a word which, according to Pococke, means a "jackal."
- The word tannin seems to refer to any great monster, whether of the land or the sea, being indeed more usually applied to some kind of serpent or reptile, but not exclusively restricted to that sense. ( Exodus 7:9,10,12 ; 32:33; Psalm 91:13 ) In the New Testament it is found only in the Apocalypse, (Revelation 12:3,4,7,9,16,17 ) etc., as applied metaphorically to "the old serpent, called the devil, and Satan."
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Smith, William, Dr. Entry for 'Dragon'. Smith's Bible Dictionary. http://www.studylight.org/dictionaries/sbd/d/dragon.html. 1901.