The pnictides MM'X, phosphides and arsenides of transition metals can exhibit very peculiar (ferro)magnetic properties. Systematic changes of magnetic ordering occur via first order or metamagnetic transitions, in such a way that marked contribution of entropy with or without magnetoelastic effects are characteristics of these transitions. E.g. for MnFeP_1-xAs_x compounds, a large magneto-caloric effect (MCE) was realized by application of moderate magnetic fields only. For a long time, we have analyzed the MM'X series for their fundamental characteristics (magnetization, neutron scattering, specific heat, Mössbauer...) as well from systematic band structure calculations. As for the parent series of mono-pnictides MnAs-MnP (M' - vacancy), we better understand the nature of the magnetic forces (polarization, exchange), that mostly proceeds via electronic instabilities. Thus a molecular field model looks not the best approximation to develop MCE analyses.