Poseidon sits next to 4 phonetic neighbors — Orion, Neptune, and Perseus are the closest matches by sound. Most share Poseidon's Greek-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"lord of the earth, husband of the earth"
"upright post, beam"
"heart, courageous heart"
"sharp, keen, pointed"
"male, manly"
"healer"
"well-pleasing, excellent"
"constant, steadfast"
"rock, stone"
"anointed one"
"virtuous, without wickedness"
"sun, throne"
"of Mars, warlike"
"first among thinkers"
"strife, discord"
Similar sound: These names share phonetic patterns with Poseidon — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Proto-Indo-European origins, giving them the same cultural heritage as Poseidon.
Rhythm: Poseidon is a 3-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.