* The most abundant animal: That would likely be the copepod. These tiny crustaceans are found in vast numbers throughout the world's oceans. Estimates suggest there could be trillions, even quadrillions of them!
* The most widespread animal: A strong contender would be the house mouse. Found nearly everywhere humans live, they adapt to a huge range of environments.
It's tricky to say definitively which is truly the "most un-rarest" since it depends on how you define the criteria. But, it's safe to say that copepods and house mice are incredibly common and widespread!