We may have such beneficial viruses inside our own bodies as well, waiting to be discovered. These viruses may not even infect our own cells but could instead be inside the bacteria that colonize us. Some species might keep the populations of their microbial hosts in check, like predators thinning a herd. Some viruses merge with bacteria rather than killing them, providing their hosts with useful genes for feeding or fighting off competitors.
I think we have bits of viruses in our DNA and RNA; did not know about whole critters.