u/Bright-Dream-9879

If you look carefully, you can see Kabuto's normal helmet. So, Atlas isn't a new rider but rather a form that give extra armor to Kabuto (and maybe Gatack?) and will probably cast off similarly to the Hyper Zecter.

I could be wrong since I'm still watching Kabuto, but that's just my opinion.

Edit: After a discussion it's probably Tendo's successor whose rider form is just similar to him. I think Toei found a way to make it relate to Tendo someway.