Undoped CoS2 is an isotropic itinerant ferromagnet with a continuous or nearly continuous phase transition at TC = 122 K. In the doped CoS1.9Se0.1 system, the Curie temperature is lowered to TC = 90 K, and the transition becomes clearly first order in nature. In particular we find a discontinuous evolution of the spin dynamics as well as strong time relaxation in the ferromagnetic Bragg intensity and small-angle neutron scattering in the vicinity of the ferromagnetic transition. In the ordered state the long-wavelength spin excitations were found to be conventional ferromagnetic spin waves with negligible spin-wave gap (\u3c0.04 meV), indicating that this system is also an excellent isotropic (soft) ferromagnet. In a wide temperature range up to 0.9TC, the spin-wave stiffness D(T) follows the prediction of the two-magnon interaction theory, D(T) = D(0)(1 - AT5/2), with D(0) = 131.7 ± 2.8 meV Å 2. The stiffness, however, does not collapse as T → TC from below. Instead a quasielastic central peak abruptly develops in the excitation spectrum, quite similar to results found in the colossal magnetoresistance oxides such as (La-Ca)MnO3
