Vykoid

The Vykoid (also known collectively as the Forgotten Army) was an advanced civilisation with a level of 18. They were only seven centimetres tall. Their small size did nothing to curb their ambition, however, and they considered the larger races in the Universe to be "lumbering brutes", thinking they were much less intellegent than them. (NSA: The Forgotten Army)

Their spaceships are built to resemble the native lifeforms of the planets they invade, and the asteroid mines of their homeworld are in reality the excrement of a huge long-extinct space animal. Their common method of invasion involves using a Time freeze device to slow down the populace, allowing the Vykoid to overwhelm and capture them. They then wire control chairs into the skulls of authority figures, in order to lure more victims. Once they have captured enough, the Vykoid teleport the captives to their slave mines. (NSA: The Forgotten Army)

History
A Vykoid army, led by General Erik, came to Earth in prehistoric times, disguising their ship as a mammoth. However, they ended up trapped beneath some ice in the Arct, perfectly preserved until 2010, when the human Sam Horwitz discovered the ship on an expedition, and brought to New York. Once there, the army reawoke, and set in motion their plans. They were foiled by the intervention of Amy Pond and the Eleventh Doctor, who altered their teleportation beam so it transported Vykoids instead of humans. (NSA: The Forgotten Army)