Equus = horse (latin) equine
Caballus = horse (latin) cavalry
Hippos = horse (greek) hippology-study of horses (a hippopotamus was literally a "water-horse")
I got these from English from the Roots up vol II. Of course, there are more words associated with each root.
The big children can use the words for copywork, handwriting, vocabulary, and spelling as they write the derivatives and their definitions and thesmaller children can draw a picture to illustrate the meaning.
We usually just take one root a week, but we took these three for the past two weeks.