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"
"son of Piers"
"well-born, youth"
"great tide"
"strong grip, paw"
"poet, philosopher, wise man"
"bright mind, shining spirit"
"born of the ocean wave"
"dark, black"
"flowing water"
"one who works the earth"
"son of Magg (pet form of Margaret)"
"greatest"
"son of Owen, young warrior"
"counsel power, wise advisor"
"flowing river, running water"
"green, dark green (water)"
"defender of mankind"
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.