What is the importance of Abraham discovering a ram in the thicket? Why not a lamb?
Genesis 22:13,14 Then Abraham lifted his eyes and looked, and there behind him was a ram caught in a thicket by its horns. So Abraham went and took the ram, and offered it up for a burnt offering instead of his son. 14 And Abraham called the name of the place, The-LORD-Will-Provide; as it is said to this day, "In the Mount of The LORD it shall be provided."
The Ram clearly represents Christ – the sacrifice that God would provide in order to fulfil his promises to Abraham. The horns of a Ram represent its strength. It was by his strength of character that Christ prevailed. It was by his strength that he endured the cross even though he could have called upon twelve legions of angels.
Matthew 26:53 Or do you think that I cannot now pray to My Father, and He will provide Me with more than twelve legions of angels?
The ram was specified in the Law for the consecration of Aaron and his sons:
Exodus 29:15-37 You shall also take one ram, and Aaron and his sons shall put their hands on the head of the ram; 16 "and you shall kill the ram, and you shall take its blood and sprinkle it all around on the altar. 17 "Then you shall cut the ram in pieces, wash its entrails and its legs, and put them with its pieces and with its head. 18 "And you shall burn the whole ram on the altar. It is a burnt offering to the LORD; it is a sweet aroma, an offering made by fire to the LORD. 19 "You shall also take the other ram, and Aaron and his sons shall put their hands on the head of the ram. 20 "Then you shall kill the ram, and take some of its blood and put it on the tip of the right ear of Aaron and on the tip of the right ear of his sons, on the thumb of their right hand and on the big toe of their right foot, and sprinkle the blood all around on the altar. 21 "And you shall take some of the blood that is on the altar, and some of the anointing oil, and sprinkle it on Aaron and on his garments, on his sons and on the garments of his sons with him; and he and his garments shall be hallowed, and his sons and his sons’ garments with him. 22 "Also you shall take the fat of the ram, the fat tail, the fat that covers the entrails, the fatty lobe attached to the liver, the two kidneys and the fat on them, the right thigh (for it is a ram of consecration), 23 "one loaf of bread, one cake made with oil, and one wafer from the basket of the unleavened bread that is before the LORD; 24 "and you shall put all these in the hands of Aaron and in the hands of his sons, and you shall wave them as a wave offering before the LORD. 25 "You shall receive them back from their hands and burn them on the altar as a burnt offering, as a sweet aroma before the LORD. It is an offering made by fire to the LORD. 26 "Then you shall take the breast of the ram of Aaron’s consecration and wave it as a wave offering before the LORD; and it shall be your portion. 27 "And from the ram of the consecration you shall consecrate the breast of the wave offering which is waved, and the thigh of the heave offering which is raised, of that which is for Aaron and of that which is for his sons. 28 "It shall be from the children of Israel for Aaron and his sons by a statute forever. For it is a heave offering; it shall be a heave offering from the children of Israel from the sacrifices of their peace offerings, that is, their heave offering to the LORD. 29 "And the holy garments of Aaron shall be his sons’ after him, to be anointed in them and to be consecrated in them. 30 "That son who becomes priest in his place shall put them on for seven days, when he enters the tabernacle of meeting to minister in the holy place. 31 "And you shall take the ram of the consecration and boil its flesh in the holy place. 32 "Then Aaron and his sons shall eat the flesh of the ram, and the bread that is in the basket, by the door of the tabernacle of meeting. 33 "They shall eat those things with which the atonement was made, to consecrate and to sanctify them; but an outsider shall not eat them, because they are holy. 34 "And if any of the flesh of the consecration offerings, or of the bread, remains until the morning, then you shall burn the remainder with fire. It shall not be eaten, because it is holy. 35 "Thus you shall do to Aaron and his sons, according to all that I have commanded you. Seven days you shall consecrate them. 36 "And you shall offer a bull every day as a sin offering for atonement. You shall cleanse the altar when you make atonement for it, and you shall anoint it to sanctify it. 37 "Seven days you shall make atonement for the altar and sanctify it. And the altar shall be most holy. Whatever touches the altar must be holy.
And for a sin offering:
Leviticus 5:15-19 If a person commits a trespass, and sins unintentionally in regard to the holy things of the LORD, then he shall bring to the LORD as his trespass offering a ram without blemish from the flocks, with your valuation in shekels of silver according to the shekel of the sanctuary, as a trespass offering. 16 "And he shall make restitution for the harm that he has done in regard to the holy thing, and shall add one-fifth to it and give it to the priest. So the priest shall make atonement for him with the ram of the trespass offering, and it shall be forgiven him. 17 "If a person sins, and commits any of these things which are forbidden to be done by the commandments of the LORD, though he does not know it, yet he is guilty and shall bear his iniquity. 18 "And he shall bring to the priest a ram without blemish from the flock, with your valuation, as a trespass offering. So the priest shall make atonement for him regarding his ignorance in which he erred and did not know it, and it shall be forgiven him. 19 "It is a trespass offering; he has certainly trespassed against the LORD.
The ram was also included in other ceremonial offerings.
I hope you find this helpful.