It would depend; mostly on the intelligence of the 100 duck-sized horses. If they used pack tactics and attacked in unison; 100 duck-sized horses would quickly overcome me and take me down, if however they acted like individuals then they would be easy to beat.
A single horse sized duck would be difficult to defeat; the feathers would act as natural armor, it would be relatively slow compared to a normal horse, but ducks are aggressive which would tip the balance in favor of the giant duck. But with pack tactics 100 duck sized horses would still pose more of a problem.