How does a (halogen) wound transformer dimmer work?
What is the equivalent circuit of a wound transformer dimmer that has only two leads?


That is a coil or inductor and is a filter to limit the amount of electro-magnetic noise generated by the dimmer. 


  I thought that the dimmer required some neutral connection too, to have a reference point for the phase control, so that the TRIAC can cut the waveform when the threshold is reached.


No, they can get a neutral connection through the lamp during the 'off' periods. There are thousands of articles on how dimmers work on the web so I don't think there is much point writing one here.