<div id="box1" onmouseover='document.getElementById("box1").style.visibility

<div id="MyDiv">This Text gets replaced with the appropriate animal_Page1.htm file based on the file name of this file</div>  
               
<script>
function getPage1() {
var Animal = getFileNameAnimal();
document.getElementById('MyDiv').innerHTML = "<iframe src='"+Animal+"_Page1.htm'></iframe>";

function getFileNameAnimal() {
//this gets the full url
var url = document.location.href;
//this removes the url end after "_"
url = url.substring(0, (url.indexOf("_") == -1) ? url.length : url.indexOf("_"));
//this removes everything before the last slash in the path
url = url.substring(url.lastIndexOf("/") + 1, url.length);

document.getElementById('MyDiv').innerHTML = "<iframe src='Dog_Page1.htm'></iframe>";

document.getElementById('MyDiv').innerHTML = "<iframe src='Animal_Page1.htm'></iframe>";