Beta decay can involve the emission of either electrons or positrons. The electrons or positrons emitted in a $\beta$-decay do not exist inside the nucleus. They are only created at the time of emission, just as photons are created when an atom makes a transition from higher to a lower energy state.
In negative $\beta$-decay a neutron in the nucleus is transformed into a proton, an electron and an antineutrino. Hence, in radioactive decay process, the negatively charged emitted $\beta$-particles are the electrons produced as a result of the decay of neutrons present inside the nucleus.