Unsoaked sugar beet.
Sugar beet needs to be well soaked in water before it is given to animals as it absorbs moisture and swells.
Dry beet will swell in a horse's gut, triggering colic or even a burst intestine.
Constable Tom Craig, of Long Eaton police, said: "It is a highly unusual incident and we think that whoever who has done this thought they were being nice to the horse, but unwittingly their actions have ended up with it being killed.
"We would appeal for anyone who may have seen or heard anything in the area to get in touch.
"We'd also ask for people to be aware about this incident to prevent a similar thing from happening again."
The owners of Ebony are very upset, he said.
The remains of a small camp fire were also found in the field.