Deformation twinning is an essential deformation mode in hcp Mg and Mg alloys, thanks to the low critical stress required to activate certain twin types and the ability to accommodate strains along the c-axis. \{10-12\} tension twins are the most commonly observed ones, and previous literature has discussed such twins being nucleated from the dissociation of <c+a> dislocations, the outcome of which usually consists of a zonal dislocation with Burgers vector several times that of a single twinning dislocation. Using molecular dynamics simulations that mimic the tensile testing of Mg nanopillars in experiments, we have observed the activation of both pyramidal I and pyramidal II dislocations, followed by the formation of \{10-12\} tension twins from the pyramidal II stacking faults. The twin nucleation occurs through pure atomic shuffling without extra dislocations being generated. Our results can help explain the experimental observation of \{10-12\} tension twins that are developed along a non-\{10-12\} twin plane at a higher strain rate.