Long live the Tardigrade!
Tardigrades, also called "water bears" or "moss piglets" are a "phylum of eight-legged segmented micro-animals (Wikipedia)." Tardigrades are almost microscopic (0.5 mm on average) and extremely resilient. They can survive extreme conditions all around the planet and can live in severe temperatures and pressures, air deprivation, radiation, dehydration, and starvation. Because of this, Tardigrades can live anywhere on earth such as the frozen peaks of the tallest mountains, the bottom of the ocean, inside of erupting volcanoes, and even outer space. They've roamed the Earth for 500 million years.
Here's a clip about these amazing creatures from the 2014 mini-series Cosmos: A Spacetime Odyssey. Tardigrades make wonderful pets, and can be found in your own backyard. Here's a guide on how to find a pet Tardigrade, care for it, and observe it under a microscope. If you're lucky, you might even see it lay some eggs while looking at it under the microscope.