The reem (plural: r'aimim) has been described differently in various sources. Bottom line, it is most likely an aurochs.
Wikipedia has a good short summary
A re'em, [has been] variously translated as a unicorn or a wild ox. It
was first identified in modern times with the aurochs by Johann Ulrich
Duerst [...] This has been generally accepted, as it is today even
among religious scholars. It has been translated in some Christian
Bible translations as "oryx" (which was accepted as the referent in
Modern Hebrew) and as "unicorn" in the King James Version, possibly
referring to a one-horned rhinoceros such as Rhinoceros unicornis.
Writing on his Talmudology blog, Dr Jeremy Brown goes through various theories: wild ox, unicorn and rhinoceros. He then concludes it is an aurochs.
The most complete treatment comes from Dr. R Nathan Slifkin in his Torah Encyclopedia of the Animal Kingdom, vol. 1 pp. 278-286. He goes through various proposed translations and explains
- that it cannot be a rhinoceros since it didn't exist in biblical lands, has only one horn and is not kosher
- that it cannot be an oryx (reem in Modern Hebrew) as the reem is a dangerous animal while the oryx is shy and elusive - although after the autoch became extinct the name reem became transferred to the oryx
- that it cannot be a buffalo as they didn't live in biblical lands
- that it has to be the aurochs, a powerful and huge wild ox that became extinct in 1627. It matches the description in various verses of being huge, having massive horns pointed forwards and upwards, being aggressive and happening to live in the relevant area
See also Theresa Bane in her book Encyclopedia of Beasts and Monsters in Myth, Legend and Folklore.