In this work we report on an experimental investigation of the nonlinear dynamics of a 850 nm multitransverse mode vertical-cavity surface-emitting laser (VCSEL) when subject to high-frequency current modulation. Different frequencies and modulation amplitudes are applied to the VCSEL. Regular periodic dynamics - with periods equal to the modulation period or twice the modulation period - and irregular pulsating dynamics are obtained. Different dynamical behaviors are illustrated by using power time traces, radio-frequency spectra and bifurcation diagrams. Our results show that irregular pulsating dynamics in multimode VCSELs subject to large-signal current modulatton can be obtained due to the competition between different transverse modes.