Rex sits next to 3 phonetic neighbors — Max, Tex, and Dex are the closest matches by sound. Most share Rex's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"king"
"herb farm, meadow settlement"
"gift, wealthy possessor"
"brave warrior (later folk etymology)"
"son of Jack"
"carried by God"
"son of Brice"
"God is my light"
"follower of Dionysus"
"one who helps"
"shining glory"
"gift of God"
"trustworthy, loyal"
"from the city of Dax"
"from Amiens (France)"
"descendant of Flann (ruddy, red)"
"supplanter's son"
"high, noble"
"he who holds firm the good"
"the king"
"wagon maker, cart driver"
Similar sound: These names share phonetic patterns with Rex — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Latin origins, giving them the same cultural heritage as Rex.
Rhythm: Rex is a 1-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.