The beings living within the mycelial network, harmed by Discovery's spore drive, who took Sylvia Tilly to the network to stop what they saw as a monster.
The JahSepp are the native inhabitants of the mycelial network, the realm of fungal life that the spore drive taps to move a ship anywhere. They saw Discovery's jumps as damaging their home and became hostile toward intruders there.
In Saints of Imperfection, a JahSepp disguised as Tilly's childhood friend May took Tilly into the network to deal with a creature that was actually Hugh Culber, preserved after his death. Paul Stamets and Tilly negotiated with the JahSepp, and Culber was returned to normal space.