The ER-3, ER-5, ER-8, and ER-10 were all experimental missiles designed to investigate hypersonic flight beyond Mach 3, and bit more info about these hypersonic tech demonstrators can be found at the following link:
The history of the Rodnik satellite network by Anatoly Zak
www.russianspaceweb.com
It's unclear whether these vehicles remained at the mockup phase only or reached the hardware phase, given that the Lockheed X-7 and X-17 provided a volume of data on aerodynamics in the hypersonic flight regime.