Theoden sits next to 3 phonetic neighbors — Theodore, Theron, and Aiden are the closest matches by sound. Most share Theoden's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"king, lord of the people"
"joined, attached"
"ruler of the world"
"man from Sebastia"
"slender one"
"rowan tree"
"hazel valley dweller"
"companion, friend"
"little fire; battle-worthy"
"anointed one"
"elder, one of authority"
"falling, rhythmic beat"
"champion, cloud"
"bright, shining"
"crown, wreath"
"stubborn one"
"resolute protector (if variant of Liam)"
"will and fame"
"man, husband (archaic)"
"white meadow"
"son of Elijah"
Similar sound: These names share phonetic patterns with Theoden — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Old English origins, giving them the same cultural heritage as Theoden.
Rhythm: Theoden is a 3-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.