the identities as we know them come along later but were certainly used by Ptolemy and most likely by Hipparchus upon whose work Ptolemy built, in particular, his table of chords.
Ptolemy made quit explicit use of the equivalent of the identities
when Hindus introduced the half-chord or sine concept, the Pythagorean identities become more obvious, and we find Varahamihira giving the verbal equivalent of...