Merlin sits next to 4 phonetic neighbors — Marlin, Myrddin, and Erlin are the closest matches by sound. Most share Merlin's Welsh cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"sea fortress"
"tile-maker, tiler"
"little dark one"
"a type of ocean fish"
"to flow, the descending river"
"deserving, worthy"
"lord, master"
"twin"
"beauty, adornment, grace"
"who is like God? (no one)"
"peaceful ruler"
"king, royal"
"lion"
"son of Carr, from the marsh"
"he judged, he governed"
"defender of men (short for Alexander)"
"divine power, God's rule"
"acquired, possessed"
"dweller at the barn"
"counsel, decision"
"elf counsel, wise counselor"
Similar sound: These names share phonetic patterns with Merlin — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Brittonic/Welsh origins, giving them the same cultural heritage as Merlin.
Rhythm: Merlin is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.