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"
"wild rose, thorny shrub"
"may God protect"
"colt keeper, young horse handler"
"from Denmark, a Dane"
"of the forest; woodland"
"wealthy guardian"
"gift, wealthy possessor"
"born at dawn"
"the cool one, refreshing wind"
"fire, flame"
"Thor's stone"
"son of the victor"
"lord, sun"
"beloved companion"
"ruler of the people"
"descendant of Madan (little dog)"
"mountain goat, to ascend"
"anointed one carrier"
"a narrow road or lane"
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.