Oh, you're on a roll tonight.
Do horse eels definitively exist, or are they an unrecognized/cryptid speicies? Personally, I agree with you that that would sound like the most likely explanation.
Cryptid. Years ago (30?) I read a book on Nessie (perhaps by Bauer, not sure about that though) which discussed this, and described attempts to find remains of a recently dead specimen which had reportedly died after getting stuck in the sluice gate in one of the old locks (not lochs, part of an old canal), in Ireland. The author(s?) was unable to find the remains, alas. But there was also a discussion (perhaps in the same chapter, or perhaps in another book) regarding various medieval accounts of the slaying of these Great Worms in various English rivers.