The Animal Capacity is a companion perk. Each level of it increses cargo of all pack animals within the expedition, up to +4 cargo at final level. However, the perk works only on animals that already have cargo capacity, thus combat animals like Hunting Dog or Hyena won't benefit from it. The perk allows to greatly expand the cargo capacity of an expedition if two or more pack animals are in it.
Native Animal Handler comes with this perk.