"Why or how were deer "introduced"? "
In the case of Royal National Park, The Deer were brought over by English settlers so they could have a familiar animal to look at and so they could have something to hunt. They also brought over plants and flowers that they liked the look of.
It still happens. A few years back,I caught a European friend of mine planting someGerman flowers in the Blue Mountains National Park. She said, 'how could anyone object to such a beautiful flower?' I should imagine the disregard for local flora was even worst in the 1800s.
Its the same with Rabbits and Foxes. They were brought over by some English settlers so they could something to hunt... originally in Victoria I believe. They're bloody everywhere now.
There's healthy population of Camels in northern Australia that are the offspring of Camel that escaped from early explorers. I say healthy because they are supposed to be the only Camels in the world which are free of certain diseases. Apparently, they are now being farmed and sent back to the Middle East and Africa.
Feral means domestic gone wild. But it gets used for pests as well.