Guinea Pig developed a ringworm on humans.

Often, children get ringworm from another child at school or on the playground. The likelihood of the guinea pig contracting it some how is not very much, since it was just home in its guinea pig cage. It is much more likely that the child contracted it from another child somewhere, and then gave it to the guinea pig while she was playing with it.
Even the dog might have been in contact with another animal and caught it, but it is very unlikely that the guinea pig did, since they don't go anywhere that they could catch it from another person or animal.