I guess it is someone who is a marxist before . Proto meaning before or early example, like prototype car. And I hhope you understand what marxist is. Maybe it is a marxist in a pre-industrialized society
'Proto' in such word-formations means first, primitive or ancestral. Proto-Marxists would, therefore, be people such as Lenin, Engels and so forth...ie the very earliest believers in communist beliefs...the 'fathers' of the movement.
Surely Proto-Marxists were Marxists before Marx had written his theories (which would exclude Lenin - he's definitely post-Marx). One of the earliest recorded examples of a Communist society would be somebody like the Apostles ("They lived together and held everything in common", Acts of the Apostles, ch 2, v. 44) - about 1800 years before Karl Marx!
'Proto' does, indeed, mean 'first in time' - as the words 'primitive/ancestral' in my earlier answer suggested. However, it can also mean 'first in rank/principal', according to 'TOED'. I'm pretty sure Lenin would be disappointed to hear he failed to qualify under that secondary heading. But what the hey!
Proto-Marxists would be those who had ideas or principles similar to Marxism, but before Marx formulated the theory. e.g. Levellers, Cromwell, kibbutzim, ancient tribal societies, etc.
Once again Bernardo comes to the rescue but this time JohnPPotts gave the best answer. You win a weekend at London Hilton. A well known proto-marxist must be those romans, a couple of brothers in the senate who wanted to divide all the lands in Rome exactly alike. Name those Bernardo