It's in the MM, and also in the Types, Subtypes & Special Abilities portion of the SRD under Improved Grab, it's been there in 3.0 & 3.5, and it's kinda important, because 1) really big creatures can grapple PCs without rolling around on the ground with them like they're play-wrestling a two-year-old, and 2) it cuts into the insane grapple bonuses such monsters tend to have:
"Improved Grab (Ex): If a creature with this special attack hits with a melee weapon (usually a claw or bite attack), it deals normal damage and attempts to start a grapple as a free action without provoking an attack of opportunity. No initial touch attack is required. Unless otherwise noted, improved grab works only against opponents at least one size category smaller than the creature. The creature has the option to conduct the grapple normally, or simply use the part of its body it used in the improved grab to hold the opponent. If it chooses to do the latter, it takes a –20 penalty on grapple checks, but is not considered grappled itself; the creature does not lose its Dexterity bonus to AC, still threatens an area, and can use its remaining attacks against other opponents. "
Now, it is an option, but, I'd expect most less intelligent creatures to do so more or less all the time (it should be instinctive, they mostly prey on smaller creatures, etc), and for intelligent ones to do so when facing multiple opponents, to avoid vulnerability.