ACo2Se2 (A=K, Rb, Cs) is a homologue of the iron-based superconductor AFe2Se2. From a comprehensive study of RbCo2Se2 via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principles calculations, we identify a ferromagnetic order accompanied by an orbital-dependent spin splitting of the electronic dispersions. Furthermore, we identify the ordered moment to be dominated by a dx2-y2 flat band near the Fermi level, which exhibits the largest spin splitting across the ferromagnetic transition, suggesting an itinerant origin of the ferromagnetism. In the broader context of the iron-based superconductors, we find this dx2-y2 flat band to be a common feature in the band structures of both iron chalcogenides and iron pnictides, accessible via heavy electron doping.