Parked Domains are pointed to the same folder as your main domain, so you have to use a script to distinguish requests to various domains.
This solution would use both .htaccess and a PHP-script.
Step 1: Create various error pages for your domains and upload them into the public_html folder:
public_html/error404-domain1.html
public_html/error404-domain2.html
public_html/error404-main.html
Step 2: Create a file named as 404.php with the code like this:
$domain = strtolower(getenv("HTTP_HOST"));
if(preg_match("/^(?:www\.)?(.+)$/i",$domain,$matches)) {
$domain = $matches[1];
}
switch ($domain) {
case "domain1.com":
include("error404-domain1.html");
break;
case "domain2.com":
include("error404-domain2.html");
break;
default:
include("error404-main.html");
break;
}
exit;
?>
This script would detect the domain and show the appropriate error page.
Step 3: Add the directive like below into the public_html/.htaccess file:
ErrorDocument 404 /404.php