The animal – spotted in Scarborough and 100 miles away in Blyth – is now thought to be heading to his natural habitat.
"Last year in North Wales, we saw animals, including snakes, bearded dragons and toads, put in very concerning situations."
Mo, who was lost while his humans were on holiday, had been living nearby all the time.
The key advice from the government is to wash hands regularly, including before and after contact with animals.
Owner of dog, now named Cracker, explained they had "no home or money now for him", the RSPCA said.
The RSPCA says nine corn snakes were found in a plastic box in Frome, Somerset.
Plastic litter incidents affecting animals are at an all-time high, according to a report by the RSPCA. The animal welfare charity found a 22% increase in incidents, from 473 in 2015 to 579 in 2018. For seals, the increase has been fourfold in the same period.
New RSPCA figures show a "shocking" increase in animal casualties.
The "horror film" discovery was made by the RSPCA on New Year's Day.