Depending on your criteria, you may find the story of Osiris interesting. In Plutarch's (wikilink) version of the story, after Set (Osiris's brother) had cut Osiris into many pieces and scattered them across the world, Osiris's wife, Isis, found all the pieces except his penis, which had been eaten by a fish.
Plutarch also states that Set steals and dismembers the corpse only after Isis has retrieved it. Isis then finds and buries each piece of her husband's body, with the exception of the penis, which she has to reconstruct with magic, because the original was eaten by fish in the river. According to Plutarch, this is the reason the Egyptians had a taboo against eating fish.
So possibly a case of castration by misadventure?
It's possible that Plutarch's interpretation was inspired by another ancient Egyptian tale - The Tale Of Two Brothers (wikilink) from which the following excerpt is most relevant:
The story centers around two brothers: Anpu (Anubis), who is married, and the younger Bata. The brothers work together, farming land and raising cattle. One day, Anpu's wife attempts to seduce Bata. When he strongly rejects her advances, the wife tells her husband that his brother attempted to seduce her and beat her when she refused. In response to this, Anpu attempts to kill Bata, who flees and prays to Re-Harakhti to save him. The god creates a crocodile-infested lake between the two brothers, across which Bata is finally able to appeal to his brother and share his side of the events. To emphasize his sincerity, Bata severs his genitalia and throws them into the water, where a catfish eats them.
As you'll note, in this one the castration is central to the story and clearly intentional.