The monster’s body is similar to a cloud of gas, or the monster can take this form. (All of the monster’s gear is absorbed into its gaseous form.) While gaseous, the monster is insubstantial and gains a fly speed equal to its land speed, as well as the ability to hover.
A monster cannot make attacks while in gaseous form unless otherwise noted, but it can move unhindered through porous obstacles that would otherwise prevent movement (such as under a door or through a cracked window).
Published in Monster Manual, page(s) 281.