Bones found in Uzbekistan belong to a creature that looked like the T-rex, but was only 10-12ft long and weighed around 600lbs.
A T-rex could be around 50ft long and weigh more than 20 times as much.
The discovery of the earlier dinosaur helps fill in a gap in the tyrannosaur fossil record that has frustrated researchers for years.
Lead researcher Stephen Brusatte from the University of Edinburgh said: “It’s one of the very closest cousins of T-Rex, but a lot smaller – about the size of a horse.
“And it comes from the middle part of the Cretaceous period – a point where we have a huge gap in the fossil record.”
The newly-discovered dinosaur – dubbed Timurlengia euotica – lived around 90 million years ago, south of what is now the Aral Sea.
It suggests that tyrannosaurs evolved relatively quickly, becoming the monstrous killing machine T-rex within 10 million years.
Timurlengia euotica’s inner ears already had tyrannosaur features associated with agility and hearing low-pitched sounds, which might have helped it detect prey at a distance.
But it lacked the massive bone-snapping teeth and large sinuses of its later relation.
The fossils found include a braincase and bones from the neck, back, tail, feet and hands.