Here is a ferret ring:
The thing on the ESC throttle cable is a 'ferrite ring'
But seriously, I'm not sure why they would fit the ferrite to one ESC but not the other. As I understand it the purpose of the ferrite is to stop electronic 'noise' from the ESC being transmitted from the ESC along the throttle wire into the receiver. This as I understand it is really only required if you are running older FM radios. Modern 2.4GHz systems operate at a much higher frequency so are not vulnerable to 'noise' from the ESC (which is at a much lower frequency). For this reason most modern ESCs dont have one.
It's interesting that the official Castle Creations photos of the Talon 25A show no ferrite fitted: