Hi there, welcome
It could be stuck on poop, but from what you are describing, I wonder if it could also be a clogged scent gland (as they can get very smelly!). Some buns need theirs cleaned out from time to time.
Does it look like what is described in this video? https://www.youtube.com/watch?v=_x9dbSUdomg
If it’s a scent gland, that video shows how to safely clean it.
If it is poop, there are a couple options. You can try breaking up the clumps with cornstarch and combing it out. If it is really matted on, a bum bath might be the easiest option. You don’t need to use shampoo. This video (by the same person
) shows how to safely give a bum bath: https://www.youtube.com/watch?v=6UcoZHBlQ80&t=10s
Did you notice him having extra, uneaten cecals? Often that can be the cause of stuck on poops when there isn’t an illness or other factor.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.