You can exactly model any arbitrary impedance network as one R and one L or C, in series or parallel. At exactly one frequency.
Whether that model is at all useful at other frequencies is beside the point.
Most often, a wider frequency range IS the point, and therefore there are a few typical models used to express things like inductors and capacitors of traditional construction.
The simplest is the series model, where you assume a resistance in series with an inductance. At DC, the reactance is zero, so the resistance must be the DC resistance (DCR). At some range of frequencies, the reactance is nonzero, and roughly proportional to F, therefore represented by an inductance.
The second and probably most useful model is the same, but with a resistor connected in parallel with the inductance. Therefore reactance rises with frequency, until it's on the order of the equivalent parallel resistance (EPR), at which point the network resembles a small resistor in series with a large resistor. EPR represents eddy currents in the wire and core, and approximates hysteresis loss too.
More and more accurate models can be constructed using more RLC components, often using ones which are frequency dependent, representing physics like diffusion and skin effect. These can be quite complex.
As for your nail core, at frequencies more than maybe 100Hz, eddy currents will dominate. You can calculate this based on the skin depth, which depends on permeability, resistivity and frequency. When skin depth is less than a third of the diameter, magnetic field is no longer penetrating most of the depth, and the core ceases to act as a core but rather a resistor. The EPR seen at the winding corresponds to the turns ratio: obviously, the core acts as one turn of whatever resistance it has, transformed up by the impedance ratio (which is the square of the turns ratio).
At 100kHz, I can guarantee the effective permeability of your core will be quite small, in the 10's, and the effective area even tinier (only a thin skin on the surface, which will be true even for very fine nails).
Tim