We have dozens of Trixie books XD It's why I mentioned her. But alas ebooks really can't be found.
I read a lot of what would probably count as "Young Adult" by some people who are paranoid about letting kids read emotional stuff. Never did me any harm. So yeah, read the real Black Beauty, but I think I had a kiddy version too. Back Beauty is PD:
http://www.gutenberg.org/etext/271
How about this? I haven't read it but it should be alright:
http://www.gutenberg.org/etext/8075
What about Louisa May Alcott's works?
http://www.gutenberg.org/browse/authors/a#a102
Also, The Jungle Book is PD:
http://www.gutenberg.org/etext/236
Oh and check out this section to PG:
http://www.gutenberg.org/browse/loccs/pz
And there is a YA and Childrens section in the ebook library here too if you hadn't checked.
(you probably have, but. In case!)