One line aspect of the film is how the main child protagonist desires for a world without traffic accidents. While considered odd by Westerners, it has its roots in the alarming number of deaths due to car accidents in Japan as industrialization increased in the 1960s. The majority of these accidents involved children, which lead to the Japanese government issuing out yellow caps to make children more visible in congested areas.
The original idea for Guiron was for it to be a flounder-like monster, with the suit-actor entering sideways. This proved too difficult to realize and was scrapped.
There were plans to use a new monster instead of Gyaos. However, the budget wouldn't allow it. It then was decided to recycle the Gyaos suit and paint it silver with the explanation that it was a different species of the creature that evolved in space.
Knifehead from Pacific Rim (2013) bears a resemblance to Guiron but this was actually a coincidence. Regardless, Guillermo del Toro was happy they made the connection, even if it was unintentional.
Aside from Guiron, a new monster called Monga was originally meant to be included. Based on a flying squirrel, time and budget limitations that it had to be replaced with the old Gyaos suit painted silver.