Artificial lift apparatus, systems, and methods for use in a deviated or horizontal wellbore, including a downhole gas separators, hydrocarbon wells including the artificial lift systems, and methods of separating a gas from a liquid hydrocarbon within a hydrocarbon well. Included is a downhole gas separator positioned in a deviated or horizontal wellbore, further including a flow-regulating device configured to restrict fluid flow through the gas outlet during at least a portion of each intake stroke of a reciprocating pump and to permit the fluid flow during at least a portion of each exhaust stroke of the reciprocating pump.