The direct answer for a Shetland pony
A Shetland pony should not eat food made for other kinds of pets, whether that is dog food, cat food, or rations mixed for goats, cattle or poultry. Every one of those feeds was balanced for a different digestive system: meat-based pet foods are far too rich for a grazing pony, and livestock rations carry mineral levels, including copper and other trace elements in proportions chosen for the target species, that do not suit a pony. What makes the Shetland's case sharper is scale. A bowl of dog food is a snack to a big horse; to a pony standing not much taller than the dog, it is a substantial, unsuitable meal.
The visitor problem: biscuits, bowls and small hands
The way a Shetland pony usually meets other pets' food is through affection rather than accident. Visitors produce dog biscuits from pockets, children share lunchbox scraps, and a feed-room cat's bowl sits exactly at pony nose height when the door is left ajar. Each individual treat looks harmless, and on a busy yard three visitors in an afternoon can quietly deliver a second lunch to a pony whose whole diet is supposed to be weighed hay. A polite sign on the stable door, naming what the pony may be given and in what amount, protects the pony without offending anyone. Carrot pieces counted into the day's ration make a far better approved treat than improvised pet food.
What the wrong food can do to a Shetland
Rich, unfamiliar food challenges a Shetland pony on two fronts at once: the gut and the waistline. A sudden load of fat and starch can unsettle the hindgut, producing loose droppings, discomfort and, in ponies prone to it, a laminitis-type foot soreness that rich feeding is known to aggravate. The slower harm is caloric. A Shetland gains condition so efficiently that regular extras undo a careful hay-weighing routine within weeks, and the crest thickens before the belly obviously changes. If a pony has eaten a quantity of livestock feed, or shows footiness, reluctance to move, reduced droppings or repeated lying down afterwards, contact a veterinarian the same day and say exactly what was eaten.
Separating feeds in a small yard or barn
Small yards make separation harder because one feed room often serves the pony, a dog, a cat and perhaps a few hens, all within a few steps of each other. Give each species a closed container rather than an open sack, feed the dog and cat indoors or behind a shut door, and sweep up spilled poultry feed, which ponies investigate with surprising enthusiasm. Keep livestock minerals and licks intended for other animals out of the pony's field and stable; a Shetland will work at a mineral block meant for cattle simply because it tastes salty. When helpers feed, one shelf per species, clearly labelled, removes the guesswork that causes most cross-feeding mistakes.
Writing the Shetland's no-extras rule where helpers see it
A Shetland pony's diet survives visitors, holiday cover and helpful neighbours only when the rule about other animals' food is written down and easy to find. State plainly on the pony's care notes: hay weight, the small approved ration, the permitted treats, and the line that no other pet or livestock food is to be offered. Add who to call if the pony is found eating something it should not. Through KinsLe, Care Teams can attach those feeding duties and the no-extras note to the Shetland's routine, so whoever is on duty, regular or stand-in, feeds the pony exactly as its owner decided.