Onyx sits next to 4 phonetic neighbors — Knox, Phoenix, and Felix are the closest matches by sound. Most share Onyx's Greek (onyx)-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"black gemstone"
"manly, strong, courageous"
"opportunity, happenstance"
"colt herder, horse keeper"
"patrician, of noble birth"
"Lord, have mercy (liturgical)"
"the victorious, the conqueror"
"great, venerable, majestic"
"poet"
"red"
"substitute, replacement"
"battle"
"from Rome, the Eternal City"
"noble and ready"
"pilgrim to Rome; from Rome"
"God is my strength"
"lord, master"
"record keeper"
"park, game preserve"
Similar sound: These names share phonetic patterns with Onyx — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Greek (onyx) origins, giving them the same cultural heritage as Onyx.
Rhythm: Onyx is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.