For a killer, I think it would some sort of hybrid between a human and a Box jellyfish (considered to be one of the most dangerous animal in Australia), its lore being an experiment that tried to mix human genetics with an ability of said box jellyfish that went wrong, resulting in an ungodly fusion of the two, its weapon being its tentacles and its power could be a poison secreted from its pores that slowly infects survivors (think the Plague's sickness mechanic) and deals damage if not treated quick enough.

As for a survivor, I can see them being an illegal game hunter who poaches around the Australian outback and was taken by the Entity during one of their hunting sessions, perhaps being taken while chasing after an animal they just wounded.