The spelling of the word "Riden" may confuse some, as it does not follow common English spelling conventions. However, the word is actually a variant spelling of the past participle form of the verb "ride", pronounced as /ˈraɪdən/ (RY-duhn) in IPA phonetic transcription. The addition of the "-en" suffix to "ride" is a holdover from Middle and Old English grammar, which used this ending to form the past participle form of many verbs. Despite its unusual spelling, "Riden" is a legitimate word that can be used in written and spoken English.
The word "riden" appears to have several possible origins, depending on its specific context and usage. It could be a variant of the verb "ride", which originally comes from the Old English word "rīdan". This Old English word, in turn, derives from the Proto-Germanic word "rīdaną", meaning "to ride, to travel".
Alternatively, "riden" could be a past participle of the verb "ride", used in some dialects or archaic forms of English. In this case, its etymology would be the same as "ride".
However, without further information or clarification on the specific usage or origin you are referring to, it is challenging to provide a more definitive etymology.