The answer is that there is no clear answer. This was debated on the OGL forums a while back.
Technically, the art in the Monster Manual is copyright WoTC. So, that's unapproachable.
However, read the SRD. When you do this, don't develop from the MM, or any other "book" - only use the SRD. Print it out, bind it, do whatever. Make that your source.
Anyway, look in the SRD - monsters have a description. Take troll for example:
SRD said:
"Trolls walk upright but hunched forward with sagging shoulders. Their gait is uneven, and when they run, their arms dangle and drag along the ground. For all this seeming awkwardness, trolls are very agile.
A typical adult troll stands 9 feet tall and weighs 500 pounds. Females are slightly larger than males. A troll’s rubbery hide is moss green, mottled green and gray, or putrid gray. The hair is usually greenish black or iron gray."
So, throw out every preconceived notion of a "troll" and create a troll solely based on this description. Do that, and you'll be perfectly fine.
Also note that certain creatures are no longer in the SRD, such as beholders, umber hulks, mind flayers, and carrion crawlers, among others.