Polygamy was obviously not part of God’s original design for marriage (Gen 2:24; also see 1 Tim 3:2, Tit 1:6), yet it does seem that polygamy was permitted under the Law of Moses and practiced by several famous OT character (e.g. Jacob, David, etc.).
To help us understand why, it is useful to look at the following passage, where the Pharisees are questioning Jesus about divorce:
They said to him, “Why then did Moses command one to give a certificate of divorce and to send her away?” He said to them, “Because of your hardness of heart Moses allowed you to divorce your wives, but from the beginning it was not so.
Matthew 19:7-9
Jesus shows that whilst divorce was allowed under the Law of Moses, it was not God’s original design for marriage. The key phrase here “because of your hardness of heart”, i.e. divorce was permitted because the Israelites were so stubborn. The same is probably true of polygamy.
One of the important things to bear in mind is that the Law of Moses was not only intended to set the moral standard but also to govern an imperfect nation. Divorce was not approved of, but it happened — there needed to be some laws to govern what to do when it did. If you look at the laws regarding divorce in the Law of Moses, none of them actually establish a law of divorce, they all concern what to do if divorce takes place. In the same way, the laws about polygamy under the law are generally about what to do if polygamy takes place (e.g. Deut 21:15f).
Also the marriageable men went off to war and died leaving behind not so marriageable men, therefore the living men took more than one wife to raise up seed. This is not Gods preferred standard it is just an example of men taking it upon themselves to provide solutions to problems instead of relying on Gods resolution.
Solomon and all his wives, he was just to rich and self centered that way and ignored Gods standard completely.
Since nearly all other forms of life in God’s creation are not monogamous, it would seem that it is simply a custom of man, and is not what God intended.
Solomon had no edict to follow.
There are verses (such as the verses quoted in the answer) which definitely suggest God’s original plan was for man to be monogamous. I don’t understand how you can decide that was “not what God intended” based on looking at other animals. Man was not created to be just another animal. He was created in the image and likeness of God. God interacts with men in a way that he just does not interact with animals. Therefore, we should listen to what the Bible says about ways in which man is created different from other animals.