Images courtesy of Peter Dorsi
The initial idea for the death knight was that of an undead lich spellcaster character mounted on a horse, much like the same character that first appeared in Warcraft II: Tides of Darkness. This concept by Blizzard contrasted starkly than the other death knights that appeared in other fantasy-based games.
However, the design and the looks of the death knight hero were revamped when Warcraft III: Reign of Chaos was released. Way different than its predecessor, this second rendition of the death knight character was rigged with qualities that reflect that of a traditional death knight. A lot of differences mark the gap between the two different generations of death knights.