Eider sits next to 4 phonetic neighbors — Aiden, Rider, and Edan are the closest matches by sound. Most share Eider's German-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"from the Eider river"
"serious, determined, vigorous"
"little warrior, young Mars"
"the bear"
"olive tree"
"work and power combined"
"uncertain, possibly Norse or Welsh origin"
"vigor and power"
"follower of Christ"
"father of peace"
"renowned warrior"
"rejoice, be glad"
"old, wise, of noble birth"
"of the Lord's day (Sunday)"
"YHWH has been gracious"
"man, husband (older Germanic sense)"
"treasurer, keeper of treasure"
"warrior"
"strife"
"venerable, revered"
Similar sound: These names share phonetic patterns with Eider — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Germanic/Nordic origins, giving them the same cultural heritage as Eider.
Rhythm: Eider is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.