Answer:
Round as a button, deep as a well. If you want me to talk, you must first pull my tail.
There are millions of me. I am very small but when moving fast I am deadly.
Two horses, swiftest traveling, harnessed in a pair, and grazing ever in places. Distant from them.
In we go, out we go. All around and in a row. Always, always steady flow. When we'll stop, you'll never known. In we go, out we go.
What is it that was given to you, belongs only to you. And yet your friends use it more than you do?
People take me on walks and I run and have fun. What am I?