É suficiente dizer que não há explicação no universo para a criatura, a não ser que exista entre os circuitos. Fora do universo, foi uma tentativa de fortalecer a tensão em uma cena que o estúdio achou "fraca".
Espero que ele não se importe comigo citando as partes relevantes na íntegra.
The stupid green monster-frog-thing was the studio attempting to "fix" what they thought was a sequence that was "not working". The circuits of time sequence was one of the first computer generated sequences in motion pictures. The entire sequence was generated without the phone booth and then the phone booth was shot via motion control and composited into the shot on an optical printer.
When the sequence was complete, the studio was (needlessly) nervous about the film as a whole and decided that the circuits of time sequence was weak, so they foolishly decided to add a "quick fix" which was the stupid green-frog-monster.