We aren’t sure what this is. It doesn’t look like any worm-like organism that we are familiar with. The next photo shows a black organism. It looks like a long piece of of black string:

This creature has some resemblance to a horsehair worm. As their name hints, horsehair worms resemble long strands of hair or pieces of thread. Adult horsehair worms are free living organisms, but horsehair worm larvae are parasites that develop in the bodies of arthropods such as beetles, crickets, and cockroaches. So, is this a horsehair worm? Let’s look at the final photo:

To us, this photograph looks like the organism from the first picture and the second picture have been combined. Going off of our first idea, we suppose this could be a horsehair worm tangled in the remains of its host. However, this seems pretty far-fetched and improbable for a few reasons. The main reason is that horsehair worms need a lot of moisture to survive, and usually exit their host when the host is near a body of water such as a pool, pond, lake, cistern, or water trough. Also, we don’t recognize the translucent organism as the typical host for a horsehair worm because we don’t know what it is. It may be possible that the horsehair worm thought it was near water and ejected itself from the host, killing itself and its host in the process, but this seems unlikely. Unfortunately, we are stumped as to what our reader has discovered on his mattress encasement. We don’t think it has anything to do with his dog, but we have no way to confirm this. We recommend he simply clean off the mattress and mattress encasement.

To summarize, one of our readers discovered a strange organism on his mattress encasement. Sadly, we weren’t able to identify the organism. If any of our other readers recognize this specimen, we invite them to comment below!