General Relativity, proposed by Einstein to describe gravitational phenomena, is a very well-tested theory at the solar system scale. However, in recent decades, new phenomena appeared, giving rise to several new alternative approaches both, on the particle content side, as well as on the gravitational physics one. In particular, the family of metric-affine theories of gravity based on the Ricci tensor (RBGs), formulated "a la Palatini'', can be put in correspondence with existing solutions in General Relativity, and vice versa. This fact makes it possible to use very consistent methods and results from the General Relativity context, to explore new results within the context of these class of metric-affine theories. This method has already been applied to several cases. In the present work we will explore the case of anisotropic fluids.