1988年11月27日在MD平台發行的《獸王記》（獣王記/Altered Beast），遊戲早期從大型機台移植到SEGA主機上，在FC及PC Engine的平台也有推出。續作《Altered Beast: Guardian of the Realms》在2002年的GBA推出。2005年在PS2販售的《獸王記》（Project Altered Beast）則變成變成3D立體模式進行。

故事描述在古羅馬的世界，地下世界的統治者Neff擄走宙斯(Zeus)的女兒雅典娜(Athena)，於是宙斯將其中一位羅馬的百夫長喚醒，並賜予威猛的力量，目的是打敗Neff救回女兒。每個關卡都有頭目，而角色會根據每個關卡分別變身為狼、飛龍、熊、虎、黃金狼。

《獸王記》總共有五關，分別為封印墓地、下水道、地底蟻洞、地下宮殿、地獄黃泉。遊戲畫面採橫向捲軸移動，每次看到白色雙頭狼，一定要快速攻擊取得靈魂球。主角要收集滿三顆靈魂球後就會變身成獸人，攻擊力量大增。通關後取得的靈魂球就會被取走。須注意雖然關卡沒有時間上的限制，但是畫面會不斷的往右快速移動，如果沒有盡快取得三個靈魂球變身，打倒頭目Neff的分身，就會持續的輪迴移動。打倒分身才會出現該關卡的真正頭目，所以最好盡量在右前方，看到白色雙頭狼就在第一時間攻擊，取得靈魂球後就可避免被漂走。

第一關的狼(Wolf)可以直接丟擲火球，跟最後第五關金色狼人的攻擊屬性一樣，但是威力更強大。第二關的龍(Dragon)可以射出閃電跟產生雷擊的屏障，第三關的熊(Bear)可以將敵人變成石頭，第四關的老虎(Tiger)可以丟擲火球。在任天堂紅白機的版本中，野獸型態還有鯊魚、獅子、鳳凰。

遊戲的主要密技有1.進入選項模式:當看到標題畫面時，先按B後再按開始紐Start，就能選擇選關，有1~5關可選(選好關卡後，回到標題畫面，按A跟開始紐Start就能成功跳關)、遊戲難度(Normal、Hard、Hardest)、血量(最多可增加到五格)、台數最多增加到五。

2.一開始可以調整每個關卡的變身，可以切換成自己喜歡的模式挑戰:標題畫面出現時，先按A、B、C紐、左下(不要放開)，然後再按開始鍵Start。

玩家可以選擇單打或雙打，雖然關卡不多並且很短，但困難度卻相當高。在90年代，《獸王記》遊戲畫面跟音樂有相當的水準，在當時造成一股風潮，算是SEGA的大作之一。

遊戲上市時，街機與MD的海外銷售數字相當亮眼，但也被部分專業機構評論關卡畫面重疊性過高及簡略。但不影響整體市場的熱愛程度，在當年有許多人是拼命的投幣試圖全破，但難度真的頗高，更別說有些不良的機台老闆們還會調整到最難模式，讓玩家們是又愛又恨。

On November 27, 1988, "Altered Beast" (獣王記/Altered Beast) was released on the Mega Drive (MD) platform. The game was originally ported from arcade to the SEGA console and also had releases on the Famicom (FC) and PC Engine platforms. The sequel, "Altered Beast: Guardian of the Realms," was launched on the Game Boy Advance (GBA) in 2002. In 2005, a 3D version of "Altered Beast" (Project Altered Beast) was released for the PlayStation 2 (PS2).

The story is set in ancient Rome, where the ruler of the underworld, Neff, kidnaps Zeus' daughter, Athena. Zeus awakens a Roman centurion and grants him mighty powers to defeat Neff and rescue his daughter. Each level features a boss, and the character transforms into different beasts depending on the level: wolf, dragon, bear, tiger, and golden wolf.

"Altered Beast" consists of five stages: the Graveyard, Underworld, Ant Cave, Underworld Palace, and Hell. The game uses side-scrolling movement, and players must quickly attack white two-headed wolves to obtain spirit balls. Collecting three spirit balls transforms the character into a beast, significantly increasing attack power. After defeating a boss, the collected spirit balls are taken away.

Although the levels have no time limit, the screen continuously scrolls to the right. If players do not quickly collect three spirit balls to transform, they will have to keep fighting Neff's doppelgänger, causing the stage to loop. Defeating the doppelgänger reveals the true boss, so players should stay towards the right, attacking the white two-headed wolf as soon as it appears to collect spirit balls and avoid being swept away.

The first level’s wolf can throw fireballs, similar to the golden wolf in the final level, but with stronger power. The second level's dragon can shoot lightning and create an electric barrier. The third level's bear can turn enemies into stone, and the fourth level's tiger can throw fireballs. In the Nintendo Famicom version, additional beast forms include a shark, lion, and phoenix.

The main cheats for the game are:

Entering the options mode: On the title screen, press B and then Start to access level selection (1-5), game difficulty (Normal, Hard, Hardest), health (up to five bars), and lives (up to five). To skip levels, return to the title screen, press A, and Start.

Adjusting beast transformations: On the title screen, press A, B, C, and Down-Left (without releasing), then press Start to switch to the desired transformation mode for each level.

Players can choose between single-player and two-player modes. Although the stages are few and short, the difficulty is quite high. In the 1990s, "Altered Beast" was notable for its graphics and music, creating a significant impact and becoming one of SEGA's flagship titles.

Upon its release, the arcade and MD versions sold exceptionally well overseas. However, some critics noted that the levels were overly repetitive and simplistic. Despite this, the game maintained high popularity. Many players invested heavily in arcade coins to try to complete the game, despite its high difficulty and some arcade owners setting machines to the hardest mode, creating a love-hate relationship with the game.