The first animal I would recommend would be the tartigrades and other extremely adaptable lifeforms. Eventually we could start to introduce insects like ants and termites. Maybe Genetically modified bees could be introduced for flowering plants that have been engineered for mars. The first land vertebrates we could introduce could be rock doves, house skinks, small rodents like mice and rats. The first large form of fauna we could release once it was pretty much inhabitable by humans could be camels and possibly some more adaptable feral animals.