Do you mean like PCP relatives? I don't know of any PCP type RCs, unless you count ketamine maybe. I did run across some phenazepam (a benzodiazepine) being sold by the gram once and as you mention, maybe some designer fentanyl once, but I never tried either.
As far as I can tell, most RCs are either tryptamines or amps/peas/cathinones. And then there are the new cannnabinoids of course, the CPs and the JWHs, and perhaps a flouro-tropane here or there. They used to sell GBL, GHB, and
1,4-BDO online too, of course, before they became more tightly regulated.
What type of drug, exactly, are you looking for?