The term "packer" is used in two different senses:
1. To refer to an equine, normally a donkey or a mule, that is primarily or only trained as a pack animal.
2. To refer to an equine that can be trusted to carry and look after literally any rider. "Packer" is a complimentary term, implying that the animal has the patience to deal with beginners and/or children, is generally non-spooky, will behave in (almost) all circumstances and knows its job with minimal guidance from the rider. Context: "That pony will just pack any kid around a course."