Full Idea
It was Aristotle who initiated the use of the letter of the (Greek) alphabet 'schematically', to stand for an unspecified piece of language of some appropriate grammatical type.
Gist of Idea
Aristotle was the first to use schematic letters in logic
Source
report of Aristotle (Prior Analytics [c.328 BCE]) by Michael Potter - The Rise of Analytic Philosophy 1879-1930 02 'Aris'
Book Reference
Potter,Michael: 'The Rise of Anaytic Philosophy 1879-1930' [Routledge 2020], p.11
A Reaction
Did he invent it from scratch, or borrow it from the mathematicians? Euclid labels diagrams with letters.