  Realizing topological semimetal states with novel emergent fermions in
magnetic materials is a focus of current research. Based on first-principle
calculations and symmetry analysis, we reveal interesting magnetic emergent
fermions in an existing material MnB2. In the temperature range from 157 K to
760 K, MnB2 is a collinear antiferromagnet. We find the coexistence of
eightfold nodal points and nodal net close to the Fermi level, which are
protected by the spin group in the absence of spin-orbit coupling. Depending on
the Neel vector orientation, consideration of spin-orbit coupling will either
open small gaps at these nodal features, or transform them into magnetic linear
and quadratic Dirac points and nodal rings. Below 157 K, MnB2 acquires weak
ferromagnetism due to spin tilting. We predict that this transition is
accompanied by a drastic change in anomalous Hall response, from zero above 157
K to 200 $\Omega\cdot \text{cm}^{-1}$ below 157 K.
