See Whitney's paper from 1935 where he defined tensor products of abelian groups (projecteuclid.org/download/pdf_1/euclid.dmj/1077490789). There you will find the terms natural homomorphism and (especially) natural isomorphism. Whitney makes no attempt to give absolutely rigorous definitions of those concepts, as the motivation to do so was lacking, but his sense of "naturality" is what Eilenberg and Mac Lane were making precise in their introduction of natural transformations.

See Whitney's paper from 1935 where he defined tensor products of abelian groups. There you will find the terms natural homomorphism and (especially) natural isomorphism. Whitney makes no attempt to give absolutely rigorous definitions of those concepts, as the motivation to do so was lacking, but his sense of "naturality" is what Eilenberg and Mac Lane were making precise in their introduction of natural transformations.