Barbaturex morrisoni, a large herbivorous lizard which lived about 40-37 million years ago during the Eocene. Known from Myanmar in Southeast Asia, it’s estimated to have reached lengths of 1.4-1.8m (4′7″-5′10″) and was closely related to modern spiny-tailed lizards.
It had a row of bony knobs along the edges of its lower jaw, which may have supported some sort of display structure. I’ve given it some fleshy double-dewlaps here, and a spiky tail similar to its relatives, but since it’s only known from fragmentary fossils these features are pretty speculative.
Surprisingly Barbaturex was much bigger than a lot of the herbivorous ungulate mammals around at the time, and was also larger than most of the local carnivores – a very different situation to modern ecosystems, where even the biggest plant-eating lizards are still smaller than ungulates.