Horses can't go through 1 block gaps, even though horses themselves are 1 blocks wide.
Linked issues
duplicates 1
Comments 2
Thank you for your report!
We're actually already tracking this issue at MCPE-14815, so I will resolve and link this ticket as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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They are not, their hitbox is larger than 1 block