Sky Pony, by Elaine Breault Hammond (Acorn Press, 2010), is the first book I've read in which a magical flying pony is the mechanism for time travel!
12-year-old Katie is fed up with her life. Not only have her parents dragged her across Canada to live in what is essentially a shack in the middle of nowhere, they have surprised her by taking in a little boy who's just been orphaned. Without consulting her. And now Siggi (Icelandic heritage) seems to be more important than she is, and she's expected to be kind to him--when all she feels is resentment.
The one good thing to come from the move is Katie's new pony, an Iceland mare named Peggy. But Peggy is more than a good pony--she can fly, and, even more magically, travel through time. Katie finds herself back in the Victorian London of her favorite doll, Samantha...and Samantha blithely welcomes her to her privileged life.
There's a dark side, though, to Samantha's London. It's a place of street crime and desperate orphans. And though Katie gets home safely after her first visit, the second time Peggy takes her back is much different. Siggi was having his first ride on her too (not Katie's idea), and soon after they arrive in London, Siggi is kidnapped and sold off, along with Peggy, to work in the coal mines. Katie, now genuinely fond of Siggi, must find and rescue them...or they'll all be stuck in the past forever.
So basically, this is an example of time travel teaching a lesson to the main character--when Katie gets back to the present, she's all about being one big happy family. Which is fine. But I must confess that I liked the beginning of the book best, before the time travel happened--Katie was very realistically unhappy (and the pony sub-plot of stable work and riding was nice!), and if the book had kept on being realistic fiction, it might actually have been a more satisfying whole.
The time spent in Victorian London was something of an improbable blur of standard street urchins speaking in dialect, and standard dark alleys, interspersed with the unreality of Samantha's life (she is, after all, a doll, so a certain unreality is expected!). But it's an exciting enough story to interest younger readers new to 19th century England.
Short answer--This is one to offer older elementary readers who want a pony story with a magical twist. Not so much one for the older time-travel fan.
Interesting detail: Katie's father lost both legs in an accident when he was her age, but went on to become a geologist, which is a nice subversion of stereotype!
(and just in case anyone notices and thinks I made a mistake-the author's name isn't hyphenated on my copy of the book, so though it is at Amazon, I didn't put the hyphen in...)