saddle grow upon him?

[keannu]

How can a saddle grow or change to a bigger one? Does it have a figurative meaning?

ex)A long time ago, a dissatisfied horse asked the gods for longer, thinner legs, a neck like a swan, and a saddle that would grow upon him....

[JMurray]

keannu.
The source of your quote (below) explains the passage. The horse considered the saddle to be a beautiful thing that it would prefer to have growing as part of its own body, rather than just being strapped on when needed. The outcome of these various changes being that the horse was turned into a camel!

Butterfly's Museum of Camels and Dromedaries
Many years ago a dissatisfied horse asked the gods for longer, thinner legs, a neck like a swan, and a saddle that would grow upon him as a part of his body. Admiring all these separate appendages of beauty, the horse longed to incorporate them all within himself.

not a teacher

[freezeframe]

How can a saddle grow or change to a bigger one? Does it have a figurative meaning?

ex)A long time ago, a dissatisfied horse asked the gods for longer, thinner legs, a neck like a swan, and a saddle that would grow upon him....

How can there be a horse that asks gods for a neck like a swan?

Strange things can happen in tales and myths.

You omitted a crucial part of the sentence:

dissatisfied horse that asked the gods for longer, thinner legs, a neck like a swan, and a saddle that would grow upon him as a part of his body.

[Raymott]

How can there be a horse that asks gods for a neck like a swan?

Strange things can happen in tales and myths.

You omitted a crucial part of the sentence:

dissatisfied horse that asked the gods for longer, thinner legs, a neck like a swan, and a saddle that would grow upon him as a part of his body.

And the reason this is necessary is that there is a figurative meaning for "to grow upon someone" that could have been inferred if this were left off.
"You know, Neddy, I didn't like that saddle at first, but after a while it grows on you."