A labrador-sized dinosaur was wrongly categorised when it was found and is actually a new species, scientists have discovered.

Its new name is Enigmacursor – meaning puzzling runner – and it lived about 150 million years ago, running around the feet of famous giants like the Stegosaurus.

It was originally classified as a Nanosaurus but scientists now conclude it is a different animal.

On Thursday it will become the first new dinosaur to go on display at the Natural History Museum (NHM) in London since 2014.

BBC News went behind the scenes to see the dinosaur before it will be revealed to the public.

The discovery promises to shed light on the evolutionary history that saw early small dinosaurs become very large and “bizarre” animals, according to Professor Paul Barrett, a palaeontologist at the museum.

When we visit, the designer of a special glass display case for the Enigmacursor is making last-minute checks.

The dinosaur’s new home is a balcony in the museum’s impressive Earth Hall. Below it is Steph the Stegosaurus who also lived in the Morrison Formation in the Western United States.

Enigmacursor is tiny by comparison. At 64 cm tall and 180 cm long it is about the height of a labrador, but with much bigger feet and a tail that was “probably longer than the rest of the dinosaur,” says Professor Susanna Maidment.

“It also had a relatively small head, so it was probably not the brightest,” she adds, adding that it was probably a teenager when it died.

With the fossilised remains of its bones in their hands, conservators Lu Allington-Jones and Kieran Miles expertly assemble the skeleton on to a metal frame.

“I don’t want to damage it at this stage before its revealed to everybody,” says Ms Allington-Jones, head of conservation.

“Here you can see the solid dense hips showing you it was a fast-running dinosaur. But the front arms are much smaller and off the ground – perhaps it used them to shovel plants in its mouth with hands,” says Mr Miles.

It was clues in the bones that led scientists at NHM to conclude the creature was a new species.

“When we’re trying to identify if something is a new species, we’re looking for small differences with all of the other closely-related dinosaurs. The leg bones are really important in this one,” says Prof Maidment, holding the right hind limb of the Enigmacursor.

When the dinosaur was donated to the museum it was named Nanosaurus, like many other small dinosaurs named since the 1870s.

But the scientists suspected that categorisation was false.

To find out more, they travelled to the United States with scans of the skeleton and detailed photographs to see the original Nanosaurus that is considered the archtype specimen.

“But it didn’t have any bones. It’s just a rock with some impressions of bone in it. It could be any number of dinosaurs,” Professor Maidment said.

In contrast, the NHM’s specimen was a sophisticated and near-to-complete skeleton with unique features including its leg bones.

Untangling this mystery around the names and categorisation is essential, the palaeontologists say.

“It’s absolutely foundational to our work to understand how many species we actually have. If we’ve got that wrong, everything else falls apart,” says Prof Maidment.

The scientists have now formally erased the whole category of Nanosaurus.

They believe that other small dinosaur specimens from this period are probably also distinct species.

The discovery should help the scientists understand the diversity of dinosaurs in the Late Jurassic period.

Smaller dinosaurs are “very close to the origins of the large groups of dinosaurs that become much more prominent later on,” says Prof Barrett.

“Specimens like this help fill in some of those gaps in our knowledge, showing us how those changes occur gradually over time,” he adds.

Looking at these early creatures helps them identify “the pressures that finally led to the evolution of their more bizarre, gigantic descendants,” says Prof Barrett.

The scientists are excited to have such a rare complete skeleton of a small dinosaur.

Traditionally, big dinosaur bones have been the biggest prize, so there has been less interest in digging out smaller fossils.

“When you’re looking for those very big dinosaurs, sometimes it’s easy to overlook the smaller ones living alongside them. But now I hope people will keep their eyes close to the ground looking for these little ones,” says Prof Barrett.

The findings about Enigmacursor mollyborthwickae are published in the journal Royal Society Open Science.