Arth sits next to 4 phonetic neighbors — Art, Arthur, and Artie are the closest matches by sound. Most share Arth's Welsh cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"bear"
"white, fair, blessed"
"son of Magg (pet form of Margaret)"
"flowing water"
"king, lord"
"high, noble, exalted"
"one who works the earth"
"poet, philosopher, wise man"
"dark, black"
"well-born, youth"
"mighty ruler with good counsel"
"rich in friends"
"great tide"
"strong lord"
"rock, stone"
Similar sound: These names share phonetic patterns with Arth — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Proto-Celtic origins, giving them the same cultural heritage as Arth.
Rhythm: Arth is a 1-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.