This is exophthalmos caused by a retrobulbar space occupying mass. It could be a tumour or an abscess, but in a rabbit that latter is far more likely. Here is an MRI of the head, showing pus extending up in the orbit and then down as far as the cervical area. The kindest thing in this case was euthanasia. Less severe lesions could be treated with enucleation and orbital exenteration, but even then the infection almost always recurs at some point.