Theseus sits next to 4 phonetic neighbors — Perseus, Orpheus, and Proteus are the closest matches by sound. Most share Theseus's Greek-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"to set, to place, to establish"
"son of Guy"
"white"
"acquired, craftsman (via Hebrew Cain/Kaine)"
"God is gracious"
"beautiful mountain"
"famous land, fame of the land"
"mountain of strength, exalted"
"shining, bright (via Hebrew Meir)"
"from the court"
"king's settlement"
"the Lord is salvation"
"of the earth mother"
"flowing, roaring (from the Rhône river)"
"gift of God"
"sea god, messenger of the deep"
"realm of the dead"
"victory of the people"
"auspicious, favorable"
Similar sound: These names share phonetic patterns with Theseus — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Ancient Greek origins, giving them the same cultural heritage as Theseus.
Rhythm: Theseus is a 3-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.