Mutant Dalek embryos were failed attempts at creating new Daleks on 1930 Earth to try to rebuild their race.
They looked vaguely like standard Dalek mutants that were made from Kaled DNA, but the lacked the eye and tentacles of their Last Great Time War mutations. The Cult of Skaro attempted to breed new Dalek mutants utilising their own genetic material, however it was not strong enough to survive the process. (TV: Evolution of the Daleks)
The resulting lifeless genetic matter was discarded in the nearby sewers, where the Tenth Doctor found one, alerting him to the Daleks' presence in Manhattan when he performed a basic DNA scan. The Cult moved on to converting humans into human-Daleks in the Final Experiment. (TV: Daleks in Manhattan)