Doc Ketchum's Horse?

I got this Collectible missing from my list... it just says boot hill reward. Am i supposrd to go back to that island and search for it or i was supposed to get it while in the mission only?
On the subject of collectibles though...

The 2 DLC collectibles, one looks like a chicken, the other looks like some sort of Fabergé Egg... If this is any hint as to what the DLC Missions are about... Then I am totally lost.
Antonio shows up with a bunch of cash, at the beginning/end, so I suspect art theft, for one.

Who knows on the chicken.
There is the rival food truck owned by "The King", which uses a chicken as their mascot?
I'm also stuck at 97% of completion. Is this 97% related to that mother %·&/* ketchum collectible?
If that one collectible is worth 3% of the entire game, then it wants to be interactable, with neon lights, and have an animation cycle that makes it run on the spot.
I checked every district, everything seems to be marked. I went to every store, every photo and collectible and i'm still Stuck at 97% and without the ketchum collectible.