Tardigrades are tough creatures which can survive extreme heat, cold, pressure, radiation etc and that's why they are appearing in Sci-Fi works these days. Here are few examples:
Star Trek: Discovery (2017) made Tardigrades famous.
In Ant-Man and the Wasp (2018), Hank Pym encountered Tardigrades in the Quantum realm. And, in the mid-credit scene, Hank Pym or his wife warned Scott Lang about Tardigrades ("Stay away from Tardigrade Fields. They look cute, but they will eat you.").
When the Tardigrades came... (2015) is the first instance I can find.
Which Sci-Fi work first showed Tardigrades?