Skrulls

"You are just one victim of the Skrull Expansion that has threatened our civilization for centuries. Impostors who silently infiltrate, then take over our planets."

- Supreme Intelligence to Carol Danvers

The Skrulls are a scientific, militaristic, and technologically advanced race of extraterrestrial reptilian humanoids that possess shapeshifting abilities.

History
"Skrulls are infiltrating your planet. They're shapeshifters."

- Carol Danvers to Nick Fury

The Skrulls were known for their shapeshifting abilities, making them difficult to recognize if they were working undercover. That led to the development of a Skrull detector technology. Shortly after the Battle of Greenwich, one black market trader on Conjunction was selling Skrull detectors for ten units apiece.

Characteristic Traits
The Skrulls are a humanoid species with distinguishable features; such as green skin and pointed-back ears.

Trivia

 * In the comics, the Skrulls are a race of alien shapeshifters bent on invading Earth as an advantage in their prolonged war with the Kree. The Skrulls managed to replace many of Earth's heroes with Skrull operatives in what was called the Secret Invasion.
 * In the Agents of S.H.I.E.L.D. episode Eye-Spy, Skrull writing appeared along the Words of Creation on a chalkboard in Todorov Building.
 * In Guardians of the Galaxy and Guardians of the Galaxy Vol. 2, the Skrull alphabet from the comics is used on various computer screens.

Behind the Scenes

 * The film rights to the Skrulls are co-owned by and Marvel Studios, similar to the rights of the Watchers, Quicksilver, and Scarlet Witch. However, certain Skrulls and Watchers characters were owned completely by Fox such as Kl'rt and Uatu.
 * The Skrulls were believed to be featured in The Avengers as Loki's army, however, Joss Whedon debunked this, saying that "The Skrulls — they can shape change. That's a whole thing. I've already got Loki. He's got magic. Once you got magic along with your Iron Man and your Black Widow — it's a real juggling act." Despite this, the Skrulls were featured in some of the films' toy line merchandise.
 * James Gunn considered implementing the Skrulls in one of the Guardians of Galaxy movies, but it did not work out.