Well, that depends on the size and weight of the toy car! If weighing a tiny Matchbook (TM) toy car, it would most certainly be in ounces. If it was a toy car a child could scoot, push with their feet, or pedal as the child rides it, it would likely weigh pounds.
The number of ounces a horse weighs depends on how many pounds it weighs. Remember one pound is 16 ounces. Equines can weigh anything from 250 pounds for a Miniature horse up to 3000 pounds for a Brabant. That means a horse could weigh in at 4,000 to 48,000 ounces depending on breed.
Technically, it depends on the temperature of the water. The density of water changes as heat is added or removed. at 32° Fahrenheit (0o Celsius), 16 fl ounces of water will weigh 1.0429756944 pounds at 100° Fahrenheit, 16 fl ounces of water will weigh 1.0359908854167 pounds