One of the weirder parts of the Mercer mythos is the importance of the donkey and the toad, "the creatures most important to him." These might be pretty ordinary animals, but in the future, they've all vanished, "become extinct" thanks to our nice little nuclear war (2.27).
Why are these two animals so sacrosanct to Mercer? It's never directly said, so we'll have to do a little inferring and see if we can't come up with an intriguing answer.
Let's start with the donkey. Donkeys are the poster animal of the "beasts of burden," creatures that have been domesticated by humans to do the difficult jobs for us. Donkeys pack heavy loads, they pull the plough, and they can even be ridden if the need arises to get from A to B. (Jesus even rode a donkey.) In short, they live, they work, and then they die.
The toad isn't worth much in the burden department. It's more work to load up a toad with camping gear than it is just to pack the stuff yourself. But these amphibians are highly adaptable. Toads live part of their lives in the water, part out of on land, and all across the world—though some of them probably shouldn't have ended up in Australia.
Based on these characterizations, we'd have to say that these animals are special to Mercer because they symbolize humanity. Humans are highly adaptable; in the novel they even moved into outer space. Yet they also have to suffer and work hard to survive in this world. Sounds like a case of toad and donkey to us.
In that case, the return of the toad—the adaptable one, represents the resurrection of Mercer back into the world through his merger with Rick. Sure, it's an artificial animal—but that's kind of the point.