Bē (ベエ) are large flies (or possibly butterflies) found in Super Mario Land 2: 6 Golden Coins and Wario Land: Super Mario Land 3. In both of these games, Bē are rather rare and somewhat unremarkable.
Bē first appeared in Super Mario Land 2: 6 Golden Coins, where they could be found in one Tree Zone level and two Macro Zone stages. These black flies typically remained in one place when undisturbed, but they, upon Mario's approaching, would quickly fly off in a diagonal manner (much like the flies of the Real World). Their runaway flight was just as fast as Mario's top running speed, making a Fire Flower or pounce from above almost completely necessary to destroy them, and even then no special reward was given for doing so, making them more or less pointless foes.
As stated above, Bē managed to reappear in Wario Land: Super Mario Land 3, likely as a clever reference to that game's predecessor. Here they were found in the first three stages of the S.S. Teacup, all of which were notable for taking place outside of the area's eponymous ship. In Wario Land, Bē's actions were for the most part the same, although Wario series mechanics made them even easier to destroy. They were also given an animation when idle and flew away in a more realistic, curved style. They would not move if Wario would crouch, which makes it possible for Wario to jump on them. If Wario manages to jump on one, a 10 Gold Coin would appear.