Allen sits next to 4 phonetic neighbors — Alan, Allan, and Alain are the closest matches by sound. Most share Allen's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"handsome, cheerful"
"man, the light"
"one who serves"
"immortal, divine"
"land of lochs, land of fjords"
"rule, sovereignty"
"water, thermal waters (place name)"
"noble bright (adal + beraht)"
"crooked nose"
"sea-born, born of the sea"
"dragon"
"dweller by the ridge"
"ruler's counsel"
"dweller by the woods"
"peace, submission to God"
"crow, dark bird"
"anointed one carrier"
"loved, beloved"
"heart, mind settlement"
"shallow river crossing"
Similar sound: These names share phonetic patterns with Allen — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Celtic origins, giving them the same cultural heritage as Allen.
Rhythm: Allen is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.