Zygons

Zygons are big, fat, lumbering, belly-button covered creatures who use holographic disguises to infiltrate alien worlds and do research for their pathetic studies. They like to bring their pets along too, which may be why the Loch Ness Monster suddenly appeared in a Scottish Loch.

They attempted to invade Earth in 1890 PLF, only to be stopped by - you guessed it - the Doctor. After that they were allowed to waddle about for a bit before flying away. Their leader, Chief Belly Button, and their scribe, Scribe Belly Button, were killed during the attempted invasion but the Zygon high council, ruled over by King Belly Button, deemed Earth too dangerous to invade again.

They tried to invade Mars but got bored and left.