In the snapshot 16w03a and previous, the hitbox of the horse was larger than the hitbox of the boat (the boat sink more and the horse's legs is out for under the boat), making it impossible to break the boat if a horse is riding passenger (bug report MC-89963). In 16w04a, it was solved thereby... If a mob's hitbox is too big it won't go into boats, the horses can not be mounted on the boat (bug report MC-96212) but, in the snapshot 16w04a / 16w05a, if you get put a horse in a boat (for example, with commands), now you can interact with the boat... The boat hitbox is below the horse hitbox (you hit the boat down and the horse will disembark because the boat does not sink and the horse's legs are inside the boat)... So, it is not necessary that the horses can not be mounted on the boat.
Test:
/summon Boat ~ ~ ~ {Passengers:[{id:"EntityHorse"}]}
and, in creative or survival, you hit at the indicated place of the images or, directly under the boat (that is the area that can be hit).
I insist on this subject because the long explorations with transport (transport without infrastructure; minecart need infrastructure: rails, bridges, tunnels, etc.) in v1.9 would be ruined.
For example:
I ride my horse, I find the sea, I let my horse, I mount on a boat alone, I arrive another continent but without a horse...
Or:
I'll mining far from home, I fill the chest of my donkey, my house is on a remote island... I can not take my donkey.
For all these reasons... I think this bug report is important, I hope this bug report not will be labeled as "Works As Intended" and I think that the solution is not that horses can not be mounted on the boats; horses and boats are not problematic.
I do not speak English well, I hope the report is understood.
Thanks to everyone and special thanks to [Mod]redstonehelper and [Mojang]ProfMobius (present in other associated bug report).
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The original report is up for review. There is no need to create new tickets for existing resolved issues.
Ok, I hoped it. It is a very nice feature for 1.9... But I will watch for it to happen! xD. Thanks 🙂
Same bug in 16w06a. I hope that the original report is up for review soon.
There is no need to keep this ticket updated.
Same bug in 16w07a/b. I hope that the original report is up for review soon. It is absurd to miss this feature.
Same bug in 1.9 Pre-release. I hope that the original report is up for review soon. It is absurd to miss this feature. I can continue this until the v1.10...
Please do not keep commenting about the same thing. You are only creating more work for us.
There is no need to keep this ticket updated.
please stop updating the affected versions, this ticket is resolved as invalid
Please reopen this at once. There is absolutely no reason to resolve this as invalid.

@unknown you can stop now repeatedly updating your "Please reopen this at once. There is absolutely no reason to resolve this as invalid." comment to have it stay on top of the activity list.
It doesn't do anything good except that you're email-spamming people that are watching this ticket and may get you banned for abusive behavior.
Please reopen this immediately. There is absolutely no reason to resolve this as invalid.
@unknown This report will not be reopened, end of story.
WHY?

@unknown, this report is a request to re-open the original report (MC-96212) about horses no longer being able to get into boats. This report is not a valid report in itself. As previously mentioned in older comments, the original report had been marked for review. For any discussions relating to the bug tracker, please do so on the Mojira subreddit.

@unknown In addition, I'd like to point out that feature requests belong in the /r/minecraftsuggestions subreddit or at https://feedback.minecraft.net ... if you think something should be changed in the game, please post there with any suggestions you have.
I think this bug report is legitimate because... The problem of hitbox (horse inside boat) solved incorrecty, then the bug persists in other "form"; the solution of first bug (hitbox) created un new bug (horse do not want to go in boats).
A "invalid" is not the solution to this new bug.
I do not think this report has been considered as it should. It has not been read carefully.
Thank you... anyway v.v.