For those unacquainted with mountain beavers, they are only distantly related to the beavers that build dams. They are also the host to the world's largest species of flea, which can reach sizes of ~1/4"!

Here is a surprisingly hilarious account of the search for a live mountain beaver flea specimen: https://www.nytimes.com/2014/07/29/science/the-great-giant-f...