why were donkeys called beasts of burden in ancient times?
Donkeys in ancient times were called beasts of burden because they were loaded down with commodities. A burden is something that weighs you down, and these animals carried such weight. The term thus describes donkeys and similar animals used to bear loads.
Les explains that in ancient times donkeys and similar animals were called beasts of burden because they were loaded down with their commodities. He notes that most people today do not know what it is like to carry a burden, which is simply something that weighs you down. He then uses this as a picture of the Law, which was a burden resting on the shoulders of the people under it. They knew nothing of liberty, only constant fear, because violations like picking up a stick on the Sabbath brought death, not a slap on the wrist. Israel was under this burden of the Law 24 hours a day until the Faith way was revealed. Les also points out that this is part of progressive revelation: God did not tell everything up front, and after 1,500 years under the Law, He progressively revealed the doctrines of grace.