Passover

Passover is represented specifically by being guilty of sin and under condemnation, but not receiving condemnation because of the substitutionary death of the creator as represented by a lamb. In the next few sections observe that the lamb represents the creator, and the animal killed by God to cover Adam's sin was a lamb as evidenced by the behavior of Abel in raising sheep.

First lamb

The first lamb mentioned in scripture is in Genesis 1:3

Ge 1:3 ¶ And God said, Let there be light: and there was light.

The word אמר 'said' is also 'lamb'. When God said, "Let there be light", he created light by the Lamb. The lamb represents the creator.

First animal killed

We don't know from this verse alone that the animal killed was a lamb.

Ge 3:21 ¶ Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them.

But from the behavior of Abel, we know it must have been a lamb.

Abel's behavior

Abel was a man who, like his brother Cain lived under the same penalty that Adam received, to eat bread by the sweat of his brow. But instead of offering God a gift of his labors, as did Cain, he raised sheep. And when he had the very best sheep, he offered it to God, because he refused to believe that he belonged in the fallen world and longed for the garden. He raised the best sheep he could as an offering to God to show that he understood the sacrifice made by the creator in covering the nakedness of his father.

It is because of his chosen profession as a shepherd that we know the animal slain was a lamb, and is the first Passover.