It's implied that Hila Tukurian became trapped in the pocket universe while exploring time travel. If that's the case how did the other creature get there?
Time travel is not restricted to humanoid species (take the dalek for example) so the creature could have ended up there the same way Hila Tukurian got trapped there. It could also be a side effect of the experiments Hila was doing or some completely unrelated experiment of the alien itself. The Doctor only said they got separated by events, war, politics or other things.