Billy sits next to 4 phonetic neighbors — Willie, Billie, and Bobby are the closest matches by sound. Most share Billy's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"resolute protector"
"flame, fiery one"
"the koa (warrior tree)"
"dark-complexioned son"
"ancient, enduring"
"great one"
"God has made, the Lord has done"
"ewe herder"
"watchful, vigilant (via Casey)"
"my God is salvation"
"emperor, ruler"
"gift from God"
"God has given"
"pure (short for Catherine)"
"God is gracious"
"drop, teardrop"
"he who causes distress"
"illuminance unit"
"long meadow, rocky meadow"
"eternal, perpetual"
"gift of God, gift of Yahweh"
Similar sound: These names share phonetic patterns with Billy — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Old German via Norman French origins, giving them the same cultural heritage as Billy.
Rhythm: Billy is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.