Wiktionary's entry for "seldom" contains the following etymology:
From late Middle English seldom, alteration of earlier selden, from Old English seldan (“seldom”), from Proto- Germanic *seldana. Cognate with Saterland Frisian säilden (“seldom”), West Frisian selden, komselden (“rare, seldom”), Dutch zelden, German selten, Danish sjælden, Norwegian sjelden, Swedish sällan, Faroese sjáldan, Icelandic sjaldan.
The expected form should be "seldon", shouldn't it? I mean, its cognates, as well as its etymon, end with an alveolar nasal. I came up with a few guesses, but the main one is:
• it was a case of "normalizing", associating with words like "kingdom" (ther isn't a lot of words in english that end with "on")
Is it right? If not, how did it happen? Are there any other examples?