In this paper, we generalize the Long-Moody construction for representations of braid groups to other groups, such as mapping class groups of surfaces. Moreover, we introduce Long-Moody endofunctors over a functor category that encodes representations of a family of groups. In this context, notions of polynomial functor are defined; these play an important role in the study of homological stability. We prove that, under some additional assumptions, a Long-Moody functor increases the (very) strong (respectively weak) polynomial degree of functors by one.