In As I Lay Dying why does Darl say Jewel's mother is a horse?

Impassioned and wild and
set apart from the family, Jewel has a special connection to a horse just as his mother has a
special connection to him.

"Jewel Bundren, Preacher
Whitfields illegitimate son. A violent young man, he loves only his horse, which costs him many
long hours of labor at night" . 

While each
child in the family is different from the others, Darl understands that Jewel is the only one
with a different father. In a figurative way, this means that Jewel's mother was "another
Addie" when she conceived Jewel -- wild, willful and untamed. Conceiving Jewel in a
clandestine affair, Addie was not the same bitter and resistant mother that she was with the
others. 

This difference is borne out in the way that Addie treats Jewel,
doting on him as she does not dote on the others, despite Jewel's hard-headed wildness. Jewel is
the one that Addie "labored so to bear and coddled and petted so and him flinging into
tantrums and sulking spells[...]."

Notably, Jewel treats his horse in
ways that directly parallel the treatment he received from his mother. He attends to the horse's
willfulness and skittishness with a passionate and whole-hearted concern, as when he sometimes
is shown "patting the horse's neck in short strokes myriad and caressing, cursing the horse
with obscene ferocity." 

Furthermore, Jewel's larger relationship to his
horse has a strong resemblance to that of his mother's relationship to him. Jewel spends months
secretly working to clear a field at night, by the light of a lantern, in order to earn the
money to buy his horse. Then he spends countless hours trying to tame the animal. The horse will
acquiesce to its situation at times, but refuses ultimately to be tamed.

The
parallel between Jewel's pre-history and Addie's narrative continues with this detail as
Addie is never resigned to her place in the family, as we find out in the chapter she narrates.
Like Jewel's horse, Addie may occupy a formal place within the family's schema but her heart is
not in it. She holds herself apart. 

"I gave Anse
Dewey Dell to negative Jewel. Then I gave him Vardaman to replace the child I had robbed him of.
And now he has three children that are his and not mine." 

Jewel's connection to the semi-wild horse that he has acquired secretly is thus a
multi-leveled parallel to his relationship to Addie, his mother. 

Finally,
Jewel is forced to give up his horse in order to complete the family's quest to bury Addie where
she wants to be buried. When he does so, Jewel's act symbolizes a recognition that his mother is
truly gone. It is a burial rite, of sorts, and one that expresses Jewel's devotion to his mother
and the degree to which he becomes forlorn at her loss.