The CT-X870IN does not have a Rhythm Editor, so there is no way to generate you own Rhythms. However, you can download User Rhythms onto the USB flash drive and load them into the User Rhythm memory slots on the keyboard. You can load AC7, CKF, and Z00 (Z zero zero) Rhythm files. Files must be in the MUSICDAT folder on the flash drive, or the keyboard will not recognize them. You will find complete instructions for loading Rhythm files into the keyboard on Page EN-50 of the manual.
AC7 Rhythm files are from the most recent keyboards. You will not find very many of those currently on the internet, but you will find an abundance of CKF files on the old Casio-Europe site:
https://music.casio.com/e/data_ex4/rhythm.html
You may find a few newer AC7 files on the Casio Music Forum site, but you will need to register as a member in order to download:
https://www.casiomusicforums.com/index.php?/files/category/39-ctk-wk/
Casio distributed the Z00 files for the WK-3300, WK-3800 and WK-8000 as CKF files, but when they were loaded into those keyboards, the keyboards converted them to Z00 files. You will find the same thing here. When you load CKF or Z00 files into the CT-X870IN it will convert them to the new AC7 format. You will see this if you ever use the Casio Data Management Software to transfer them back to the hard drive on your computer. They will all come back as AC7 files.