To reality and beyond

How do you categorize a story that's true but not true? Snoopy's story is true, in the timeline, historical sense. He was born on the ranch, raised in pens with different horses, started with the groom, shown by Tina, broke his leg, etc.

But when I wrote his book, I took many liberties with the characters, especially those in the hospital. He was there for a long time. I don't know how he passed the time. So the horses he encounters there are completely fictional.

When it came time to write that first page of the book that says, these characters are fictional, I was in a pickle. How to I write that it was true and not true?

Here's what I went with:

This is a work of fiction, based upon a true story. Some of the characters are fictional. Even the characters that truly exist have been modified to suit the author’s needs. Unless you believe that horses really talk. Then it’s all completely true.

Think that works?