It comes from interdimensional portals between Tamriel and Post-War America.
Did we not already go through this before? :laugh:
@OP:
Squirrels and Iguanas might just be those "background characters" that you never see in Fallout. A lot of games have these characters that play some role, but never actually is encountered by the PC.
I would have come with an example, but I'm tired right now, so I cannot be asked :laugh: