I saved a picture (cant find it) of when the Thunderlizard was mentioned in either 5th or 6th edition (maybe?) Can anyone help me out? It was mentioned with the Carno and Coatl. Thanks! - Suds
Thunder Lizard wasn't mentioned in 6th at all as far as I can tell, though I don't know about 5th. However I think you might be thinking of the 2004 Warhammer Chronicles book, where they have rules in a special "no Lizardmen" army, as do Carnosaurs, Stegadons, Coatl and Great Wyrms and some other dinosaurs.
Neat it has rules. I thought it only existed in the lore. Did some searching, here is what the cover looks like: https://whfb.lexicanum.com/wiki/Warhammer_Chronicles_(2004)