Answer
Snowball is described as "a more vivacious pig than Napoleon, quicker in speech and more inventive, but was not considered to have the same depth of character." He is also said to have "a reputation for getting his own way."
Napoleon is described as "a large, fierce-looking Berkshire boar, the only Berkshire on the farm, not much of a talker, but with a reputation for getting his own way."
Squealer is described as "a small, fat pig with very round cheeks, twinkling eyes, nimble movements, and a shrill voice." He is also said to be "a brilliant talker" and "could turn black into white."
It is possible that Snowball and Napoleon were not introduced earlier because they were not yet relevant to the story. Their importance now is due to their roles in the power struggle that develops among the animals.
The inciting event that moves the story from exposition to rising action is Old Major's speech, in which he encourages the animals to rebel against their human oppressors.
The Seven Commandments are:
1. Whatever goes upon two legs is an enemy.
2. Whatever goes upon four legs, or has wings, is a friend.
3. No animal shall wear clothes.
4. No animal shall sleep in a bed.
5. No animal shall drink alcohol.
6. No animal shall kill any other animal.
7. All animals are equal.