Q:

How can I use dynamic react class element in render?

I understand the question is not clear, so the description will help. 
So, I have some react components like,
var LandingPage = React.createClass({
    render: function() {
        return <div>
            This is the landing page.
        </div>
    }
})

and another component like 
var FirstContent = React.createClass({
        render: function() {
            return <div>
                This is first content page
            </div>
        }
    })

Now I have a controller where I decide which one I need to render by passing a value in props, something like this - 
var Contoller = React.createClass({
        render: function() {
            var inside = "";
            if (this.props.pageName == "LandingPage") {
                inside = <LandingPage />;
            } else if (this.props.pageName == "FirstContent") {
                inside = <FirstContent />;
            }
            return <div>
                {inside}
            </div>;
        }
    })

Now instead, I want to do something like, use the this.props.pageName inside the tag directly, so that I don't have to write if else for every time ad some new alternate content. Should be something like this - 
var Contoller = React.createClass({
        render: function() {
            return <div>
                <"this.props.pageName" /> //comment - <LandingPage /> if this.props.pageName = "LandingPage"
            </div>;
        }
    })

A:

The map of pageName to actual Component has to exist somewhere, because other than the default HTML elements (like div) React needs the class object reference to render a component. A string wont do.
How you manage this map is up to you, but I've used an object below.
Further complicating this is the JSX compilation step, which doesn't work with dynamic content. You will have to use the actual JS calls in your Controller to get this working.
Here is a codepen demonstrating this.
class LandingPage extends React.Component {
  render() {
    return <div> This is the landing page. </div>;
  }
}

class FirstContent extends React.Component {
  render() {
    return <div> This is the first content page. </div>;
  }
}

const map = {
  LandingPage: LandingPage,
  FirstContent: FirstContent
};

class Controller extends React.Component {
  render() {
    return React.createElement(map[this.props.pageName]);
  }
}

React.render(<Controller pageName={'LandingPage'} />, document.body);

All that being said, I think you are building a router. You can use react-router in memory mode to perform routing without using the URL. Rolling your own setup here may be more work than it's worth.

