E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Certain Scotch bishops appointed by James I., with the distinct understanding that they were to hand over a fixed portion of the revenue to the patron. A tulcan is a stuffed calfskin, placed under a cow that withholds her milk. The cow, thinking the tulcan to be her calf, readily yields her milk to the milk-pail.