Hello Raymondo,
Thank you for the details of the issue you are experiencing with transferring videos to your iPod classic. As you suspected, HD videos are not supported on the iPod classic:
HD videos purchased from the iTunes Store are offered in 720p or 1080p and are compatible with iPhone 4 or later, iPad, iPod touch (4th generation or later), Apple TV, and computers. You may need to download your purchased video from iTunes in the Cloud in a lower resolution for your device.
Use the steps in the section titled "Download previously purchased content in a lower resolution" in the following article to download the Standard Definition version of your videos:
iTunes: Sync purchased HD videos to iPod or iOS device
http://support.apple.com/kb/TS4223
Thank you for using Apple Support Communities.
Best,
Sheila M.