A supplement for Monster's Handbook.
From the introduction:
Combining magical abilities with the sheer ferocity of a mighty animal, magical beasts are often drawn from mythology, folk tales, and other sources to threaten characters in fantasy
games. In most cases, a magical beast is merely a wild animal that through a trick of magic has gained abilities not normally possible in the natural world. Some can breathe fire, others can petrify their opponents, and still others can slip into the ethereal plane at will. Magical beasts make good opponents because of their combination of combat skills and magical abilities allow them to handle a wide range of party abilities. Most of these creatures have abilities that enhance their prowess in melee, posing a difficult challenge for both the characters’ abilities and the players’ problem solving skills. Best of all, most magical beasts are either classic gaming monsters or inspired by myth. They resonate with players, as their appearance and background firmly cast them as powerful, vicious monsters.