Usually, the starting point of the expression in the English literature, is
Marshall's inaugural lecture in 1886 (but I never checked this). Pareto's
first mention to homo oeconomicus is a letter to Pantaleoni in 1891, where
he wrote: "can you tell me who was the first to use the expression 'homo
oeconomicus"?, a question clearly still up to date.

But the "concept" or the methodology of "homo oeconomicus" goes back at
least to the young J.S. Mill (1836). There are many papers on this, included
Guala and mine, HOPE 2002, on the relationship between Pareto and Mill on
this point.

Luigino Bruni