Rex is from the Latin rex (king). Oedipus Rex (c. 429 BCE) — Sophocles's Greek tragedy; the title means "Oedipus the King"; Aristotle's Poetics analyzed it as the perfect tragedy; the source of Freud's Oedipus Complex (covered separately under Jocasta); performed continuously for 2,500 years. Tyrannosaurus rex — iconic prehistoric dinosaur.
Rex does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Rex reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.