Companion animals are a source of CO2 in proportion to their size and diet. Big dogs being the most polluting. Cats eat wild birds, rodents and amphibians. A pet rabbit is not so bad and you can eat the rabbit if needs be. All that said I think a companion animal can be a good thing for lonely old people. If you take into account transporting stuff and other non-direct CO2 then probably the worst offenders are horses, many of which "require" a Disco or similar for towing which then gets used for other driving.
Rebreathers are like women; they pretend to love you, whilst taking all your money and trying to kill you.