Realizing topological semimetal states with novel emergent fermions in magnetic materials is a focus of current research. Based on first-principles 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 network close to the Fermi level, which are protected by the spin group in the absence of spin-orbit coupling. Depending on the Néel 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 ∼200ωcm-1 below 157 K.