Acro is a boss character that appears in Kirby's Dream Land 3 and Kirby 64: The Crystal Shards. He is a large orca whale that apparently lives in a deep sea trench in Aqua Star. He is not entirely evil, however, as he was under the influence of Dark Matter mischief. His name is in fact "Orca" spelled backwards, and short for acrobat, which describes the swift flipping motions he makes while fighting Kirby underwater.
As stated in the Kirby 64: The Crystal Shards Prima guide, Acro's name comes from the word acrobat. His name is also orca backwards, a reference to his dolphin species.
Acro was designed based on a female killer whale. This can be clearly seen when pointing out certain traits such as the dorsal fins. While Japanese sources do not give gender-specific pronouns and the games clearly show Acro spawning baby whales, the only official English source (the licensed Prima guide) indicates that it is male.
In the instruction manual for Kirby's Dream Land 3, Acro is listed as "Akro the Whale". This is most likely a spelling error, as his name is spelled correctly in the game.
In Kirby & The Amazing Mirror, a boss named Gobbler is believed to be the Mirror World counterpart of Acro.
In the proposal documents of Kirby's Dream Land 2, an orca resembling Acro appears in a drawing showcasing the game's various characters (which also appears in the booklet included with Kirby's Dream Collection: Special Edition), yet this orca never appears in-game. This may hint that Acro was originally intended to appear in Kirby's Dream Land 2.
According to a pre-release screenshot's description, Kirby's inability to assimilate (or inhale) aquatic predatory mammals (ie. Acro) stems from a glitch in Kirby's DNA.
Acro Bot, an enemy from Kirby: Planet Robobot, is likely based off of Acro's name and appearance.