Yes, there was polygamy in OT times (at times). But Abraham was NOT a polygamist.
The lone reference to Hagar being a "wife" of Abraham was his wife, Sarah, saying he would give him her servant for childbearing purposes.
After she was pregnant, all the references to Hagar are NOT as a "wife," but still as Sarah's servant.
Abraham references her that way; so does the Angel of the Lord; so does Moses (who wrote Genesis); so does the apostle Paul in Galatians 4:21-31.
Abraham took another wife after Sarah died.
Jacob was a polygamist; but that was something that developed directly due to deception. Solomon was a polygamist, but that was heavily frowned upon in one of the books of Kings -- essentially proving Deut 17:17 -- that polygamy would take away a man's devotion to the Lord.
Some of the other OT mentions of polygamists involved people who were hardly "models" of the Lord. We know David had wives and concubines; some of the concubines he simply inherited when he became king...we don't know if he slept with any of them. We know some of David's wives were not simultaneous to others.
David and Solomon were both polygamists. Don’t get me wrong, I don’t think polygamy makes sense given the way social benefits go nowadays, but David’s real sin was conspiracy to murder someone to get a woman who was already married to him. He had plural wives at that time, that is why Nathan told him the Ewe lamb allegory, why couldn’t he be satisfied with the wives that he had instead of kill a man to get his wife? Additionally Solomon was super-polygamous, but was wrong for marrying polytheist wives and practicing their religion with them. Jacob had two wives (Leah and Rachel) and two concubines. No need to try and get a little revisionist with what goes on here. Not all these people in the Bible who practiced polygamy were bad, and often it wasn’t just the polygamy that made them bad at the same time. Stuff happens, I think living in the ancient world, and the conditions back then made polygamy more permissible. Today, it’s easier to do half a dozen other things than to practice polygamy; it’s really impractical unless you are a pretty rich guy, or don’t mind the child custody legal standings associated with marriage.