Green’s Dictionary of Slang

croakus n.

[crocus (metallorum) n. (1)/croaker n.5 (1)]

a doctor, esp. a quack.

[UK]Hotten Dict. of Modern Sl. etc.
[UK]B.M. Carew Life and Adventures.
[US]W. Edge Main Stem 137: The reminds me of the man that the croakuses (doctors) killed over at the Thor Company.