I disagree. Horses don't look happy. Horses don't smile and they're not cute.
This horse has an important role (carrying the sun child, the newborn sun) and is aware of it.
The white horse has long been a symbol for paradise and a noble spirit (and does so until today, I'm reminded of Tolkien's
Shadowfax, played by a white horse in the movie).
Here are some important depictions of white horses, underlining their association with light (even divine light), mythological creatures like unicorns, and paradise:
Jan Breughel,
Velazquez,
George Stubbs,
John Constable,
Gauguin and
Franz Marc.
Portraying horses' faces has a long tradition in art. Especially expressive:
Gericault,
Toulouse Lautrec.
Colman-Smith studied art and whether she knew these paintings (all of them very well-known) or not, she was certainly aware of the long and important tradition of horse paintings. No, I don't think Colman-Smith wanted a cute, smiling horse face. She depicted a dignified, strong horse. She was a good enough artist not to add, like
cartoon artists, eye brows and a smile. That's nice for a children's movie (which I like very much), but it's absolutely not her style.