  The subclass of collinear antiferromagnets where spin Kramers degeneracy is
broken -- so that they can display ferromagnetic-like properties -- stimulates
a new perspective in magnetism. However, the material base for these so-called
altermagnets is still relatively limited. We show that Ruddlesden-Popper and
perovskite phases generically host altermagnetic materials. In particular, we
analyze prototypical nickel-based systems via first-principles calculations and
identify additional materials from their crystal structure and magnetic orders.
These materials span both insulating/semiconducting and metallic conduction
types and the computed values of the nonrelativistic spin splittings reach
250~meV. Our analysis also indicates some formal inconsistencies that may
appear within the traditional classification of magnetically ordered materials,
which can easily be resolved from the altermagnetic perspective. In addition,
we indicate the possibility of having altermagnetic behavior in systems beyond
collinear antiferromagnets with perfectly compensated magnetization.
