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"
"night"
"abbot, religious father"
"foreigner, of the Welsh"
"fort, castle"
"modern coinage"
"lion of Judah (royal, courageous)"
"auspicious, favorable"
"courteous, well-bred, polite"
"bold journey, brave traveler"
"place of protection and calm"
"fast sailing vessel"
"gift of God"
"related to Everett (brave boar)"
"YHWH has shown favor"
"noble bear"
"thin, lean"
"one of free status"
"child born at a prosperous time"
"powerful giant, cosmic force"
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.