In the process of negative beta decay,

In negative beta decay ($\beta^{-}$ decay), a neutron inside the nucleus is unstable and transforms into a proton, an electron, and an electron antineutrino.

The nuclear reaction is given by:
$n \rightarrow p + e^{-} + \bar{\nu}_{e}$

The emitted electron ($e^{-}$) is expelled from the nucleus as a beta particle. Electrons do not pre-exist inside the nucleus; they are created during the decay process.

Therefore, a neutron within the nucleus decays and emits an electron.