What's the fastest way to copy the values and keys from one dictionary into another in C#?

There's the Dictionary constructor that takes another Dictionary.

You'll have to cast it IDictionary, but there is an Add() overload that takes KeyValuePair<TKey, TValue>. You're still using foreach, though.

There's nothing wrong with a for/foreach loop. That's all a hypothetical AddRange method would do anyway.

The only extra concern I'd have is with memory allocation behaviour, because adding a large number of entries could cause multiple reallocations and re-hashes. There's no way to increase the capacity of an existing Dictionary by a given amount. You might be better off allocating a new Dictionary with sufficient capacity for both current ones, but you'd still need a loop to load at least one of them.

var Animal = new Dictionary<string, string>();

one can pass existing animal Dictionary to the constructor.

Dictionary<string, string> NewAnimals = new Dictionary<string, string>(Animal);