Selachian

The Selachians were the Ockoran soldiers in the Selachian Empire. They had been specially modified for war, having their lower halves replaced with legs and being installed in an armored shell. (PDA: The Final Sanction)

Technology
The Selachians were distinguished by the Selachian battlesuits. These battlesuits were large, bulky things that gave the Selachians the appearance of humanoids. Selachians had to have studs implanted into their nervous system and have their tails removed in order for them to use the battlesuits. They had plasma weapons attached to their arms, which were built for fear rather than power (though it was still a dangerous weapon). Their helmets were shaped and painted to look like sharks in order to intimidate their enemies. Later battlesuits were equipped with nanites to repair damage and to counteract the nanites of the Terran Security Forces. (PDA: The Final Sanction)

Culture
The Ockorans adopted the name "Selachians" after studying other worlds and learning that sharks were the most commonly feared marine creatures amongst humans. (PDA: The Murder Game) Selachians had a strong sense of pride and would refuse to show weakness or fear. They would rather die than give into their enemies. Conversely, their weaponry, armour and name was specifically chosen to cause fear in the enemy. Selachians hierarchy was shown by stripes painted onto their battlesuits, going from top-left to bottom-right. The Supreme Leader's suit had black stripe edged with red, while the next level down had blue stripes. (PDA: The Final Sanction)

History
At some point before the 22nd century the Ockoran's home-planet of Ockora was discovered by the Kalarians and chosen as a vacation spot. Though the Kalarians knew of the Ockorans, they believed they weren't intelligent and hunted them for sport. (PDA: The Final Sanction)

The Ockorans eventually developed armour that allowed them to breathe on land, and emerged from the oceans during a culling festival and took their revenge on the Kalarians. Their homeworld was cordoned off and the Ockorans were eventually forgotten. (PDA: The Murder Game) The Ockorans developed space travel and left Ockora. In order to take their revenge on the Kalarians, they followed them to their homewolrld of Kalaya and conquered the planet within the six months. They then conquered three more neighbouring worlds within a year. The Selachians then formed the Selachian Empire, and soon became a major power on the galactic markets. (PDA: The Final Sanction)

In an alternate universe, the Selachians attacked Moonbase in 2044. (BFA: The Architects of History)

By 2069, the Selachians had encountered humans. They were featured in the Eye-Spy Book of Alien Monsters, which was handed out to UNISYC personnel as potential threats. (EDA: Alien Bodies)

In 2146, the Selachians attacked Hotel Galaxian. (PDA: The Murder Game)

In 2191, the Cybermen stole a Selachian ship. (MA: Killing Ground)

At some point before 2211, the Selachians invaded the Earth colony Rho Priapus. The planet was later taken back by the Space Marines. (EDA: The Janus Conjunction)

For decades, the Selachians and the humans had traded with each other. Eventually, towards the end of the 22nd century, one deal went particularly bad, causing the humans to declare war on the Selachians. The war went badly for the Selachians and by 2204 they had been pushed back to Ockora. Though they had taken prisoners on planets they were forced to leave, such as Molinar and Kalaya, this wasn't enough to save them and Ockora was destroyed by General Wayne Redfern when he dropped the G-bomb.

After the meddling of the Second Doctor, the Supreme Leader and forty Selachian soldiers were able to escape the planet's destruction and get on-board the Triumph. They killed everyone on the ship, but another G-bomb was accidentally set off, leading to the formation of another black hole and killing the last of the Selachian race. (PDA: The Final Sanction)

In 2775, Jack Harkness told a story about when he met the Selachians. (NSA: The Stealers of Dreams)