The lamb is a spiritual 'animal.' It's not physical... it's not from a flock of sheep. It's spiritual, and the whole scene is in spirit. Neither is it human.
When John (the baptizer) saw Jesus coming toward him, he declared- "Behold the lamb of God." But just as Jesus was not a high priest on earth, neither was he a lamb on earth. It's the heavenly reality that we need to keep in mind. In the heavenly realm, Jesus IS a high priest and in that role he is the one offering this lamb as a gift and sacrifice to God. It's this scene that John has some personal knowledge of, just as in spirit he had previously seen the spirit descend (like a dove) and rest on a man, whom he later recognized as Jesus. A dove. A lamb. A lion. Jesus was none of these, yet these are various spiritual attributes given as spiritual representations -- spiritual concepts that we can then associate with the spiritual nature of things. If only Moses had made these associations with the spiritual things he had seen, but he didn't. A whole generation of people were led astray, wandering in the wilderness, going in circles-- and Moses himself denied entry into the promised land. But Moses had no understanding of these spiritual creatures.
In the middle of the throne and around the throne were four living creatures full of eyes in front and in back. The first living creature was like a lion, the second creature like an ox, the third creature had a face like a man’s, and the fourth creature looked like an eagle flying. Each one of the four living creatures had six wings and was full of eyes all around and inside. They never rest day or night, saying:
“Holy, Holy, Holy is the Lord God, the All-Powerful,
Who was, and who is, and who is still to come!”
And whenever the living creatures give glory, honor, and thanks to the one who sits on the throne, who lives forever and ever, the twenty-four elders throw themselves to the ground before the one who sits on the throne and worship the one who lives forever and ever, and they offer their crowns before his throne, saying:
“You are worthy, our Lord and God,
to receive glory and honor and power,
since you created all things,
and because of your will they existed and were created!
These 24 "throw themselves to the ground"-- and offer "their crowns" -- They leave their home above, giving up their lives (the crowns they had been awarded) as an offering, in service to the one upon the throne. And they do so willingly, with no fear of the second death, which can never harm them. They have already, previously conquered over death and they can lay down their life and take it up again.