The Brussels Airlines pet policy is as follwos:
Pet are accepted in the aircraft cabin and in the hold.
Only cats (max. 8kg/18 lbs, including the pet bag) and dogs (max. 8kg/18 lbs, including the pet bag) are accepted in the cabin.
All animals, other than cats and dogs, must be shipped as cargo in accordance with the IATA Live Animals Regulation: Rodents, Rabbits, Fish, Birds, Large domestic animals (> 8kg), Wild animals
Pets are not accepted in the cabin on Brussels Airlines flights to the United Kingdom. However, cats, dogs and ferrets are accepted on flights to London Heathrow under the UK Pet Travel Scheme when shipped as Cargo.
Travelling with pug-nosed dogs and cats (e.g. pugs, bulldogs, boxers, Pekinese and all cross-breed as well as Himalayan, Persian and exotic short-hair cats) as Pet in Cabin (PETC), Live Animals in Hold (AVIH) or as Cargo can be accepted upon agreement.
Taking a pet to a foreign country involves preparation and planning far in advance of your departure date. Requirements for pet travel to a foreign country are set by the destination country. If you are you traveling to another country and bringing along your pet you’ll need to meet the animal health requirements of the country you are visiting.
More information on the Brussels Airlines pet policy