We present results of a two-dimensional hybrid simulation of the electromagnetic proton beam instability. We show that for a case of cold and rather high density proton beam the oblique right-handed resonant modes play an important role. The interaction of these modes with the beam protons causes significant beam density fluctuations. At the nonlinear stage the oblique modes give rise to a beam filamentation. Beam filaments influence strongly the evolution of the instability. They create pulses of a strong magnetic field. These pulses share some properties of the magnetic pulsations (short, large-amplitude magnetic structures). We suppose that the oblique modes play an important role in the structure of the quasi-parallel Earth's bow shock. ¿ 1999 American Geophysical Union