All the Pretty Horses gets its title from a lullaby of the same name. There are slight variations in the title and lyrics of the lullaby, but the key passage goes a little something like this:
Hush-a-bye, don't you cry
Go to sleep, my little baby
When you wake, you shall have
All the pretty little horses
Dapples and grays, pintos and bays
All the pretty little horses! (Source)
The choice of a nursery rhyme is, we think, pretty weird. It's never explained or referenced in the novel, and it may seem difficult to connect to teenage rebellion and coming of age, gunfights, and wandering the badlands as an outlaw of sorts.
Our theory is that there may be an intent to be ironic on the author's part, given the lullaby's emphasis on dreaming and childhood compared to the harsh environment of the novel. The closest the novel gets to such an idyllic collection of horses is the early scene on Don Héctor's ranch where John and Rawlins break a pack of wild horses, but this moment fades in the wake of the violence and romance to come.