This article is from the Xena: Warrior Princess FAQ, by Paul Hilling p.hilling@bigfoot.com with numerous contributions by others.
Well, it probably had to do with the fact that it is the XENA:
WARRIOR PRINCESS show not the X & G show. Gabrielle started out
as Xena's tag-along, not an equal. They even made it clear that
Gabby didn't like horses. There have also been several fan
discussions regarding Gabby's apparent fear of heights. The
thought being that it may not be so much that Gabby doesn't like
horses, as much as the fact that she doesn't like tall horses.
Remember the comment Gab made that went (something) like this?
"When they make them [horses] closer to the ground..."
Things are changing though. In "The Greater Good", Gabrielle and
Argo "make friends". Gabrielle finally confesses to Xena that
she had once had a pony, which died, and after that time she was
uncomfortable around horses and ponies.
In "A Solstice Carol" Gabrielle acquires a donkey, but she ends
up giving the animal away to a couple with a child.
In "For Him The Bell Tolls," Gabrielle is forced to dismount Argo
(she was riding behind Xena) and stay behind when Xena goes off
on some mysterious mission, giving Gabrielle and Joxer a show of
their own.
For the reality-based reason why Gabrielle didn't have a horse,
see Question IV.4.
In an online chat RJ Stewart (One of Xena executive producers)
said that they were looking to give Gabrielle a horse in Season
5, and we see this coming to pass in the Season 5 episode "Animal
Attraction".
 
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