Lisowicia Bojani was a giant double fanged relative of ours that lived in the late Triassic, when dinosaurs were plentiful, but mostly hadn't gotten gargantuan yet. But Lisowicia was big – nearly the size of an elephant.
Lisowicia was a type of therapsid called a dicynodont. You're a therapsid and I am a therapsid, and therapsids would meet most peoples' impressions of what a mammal was – it likely lactated, was warm blooded, and almost certainly had hair. (What actually makes you a mammal, technically, isn't these things, but the fact that bones in reptile jaw joints evolved into our inner ear, a process that was still underway and incomplete in earlier therapsids.)
What is so unusual about this as that creatures like us weren't supposed to be big like this in the late Triassic. Mammals and relatives had been chased into the shadows by the super-efficient metabolisms, respiratory systems, and gaits of the dinosaurs. This change had been underway since the beginning of the Triassic, when therapsids, gravely set back by the Permian extinction, were not able to hold onto their dominance in the ecosystem.
But Lisowicia, whose discovery was announced by Tomasz Sulej from the Polish Academy of Sciences and Grzegorz Niedzwiedzki from Uppsala University in Sweden, was 4.5 meters in length, 2.6 meters tall, and weighed nine tons. That compares evenly with modern African elephants. And Lisowicia, like modern mammals, walked with its legs directly underneath it, and not sprawling like reptiles and amphibians.
Dicynodonts show all the signs of being warm blooded. They had the levels of “vascularization” (presence of veins) associated with mammals, and the young were more vascularized than even we are today. Coprolites (prehistoric turds) of animals that ate dicynodonts usually contain hair, which is how we came to think they were covered in hair like us.
Therapsids prevailed in the late Permian, which in many respects can be thought of as the “first age of mammals”, in that then, as now, our ancestors were dominant on land. The end of the Permian nearly killed all life on Earth, and although one dicynodont (Lystrosaurus) was literally the only land animal that thrived through that extinction, the Triassic cost we therapsids our dominant position in the food chain, and dinosaurs took up the apex position for the next hundred million or so years.