A lot of generic cards use models from the game but with different names. For example, Mechwarper is just some Ulduar minion and Lil'Exorcist is just a normal gnome priest that could exist anywhere in WoW.

The only completely unique card that comes to my mind is "Annoy-O-Tron". He's completely 100% a Hearthstone creation (New raidboss for next patch? pls blizz)

Edit: Mech-Bear-Cat and Enchance-o-Mechano are also found nowhere in WoW.

Edit 2 - Electric Boogaloo: There are also no Metaltooth Leaper, Goblin Auto Barber, Iron Sensei, Whirling Zap-o-Matic, Shadowboxer, Screwjank Clunker, or Cobalt Guardian models in-game.

Edit 3 - The editest edit: As far as I can tell vanilla Hearthstone did a very good job of including only in-game NPCs. The only two vanilla cards I can think of that has 0 counterparts in WoW would be Lightspawn (Many similar elementals exist - but no elementals of light) and Tundra Rhino (No winter Rhinos in any of the snowy zones in the game)