Complex sentence: Perhaps a little less likely is the notion that Brockman is also an occupational name among families which may have specialized in wild brock (horse), and in breaking (Middle English: broke and broken, to break and to have broken) brock horses.
Split into simpler sentences: Perhaps a little less likely is the notion that Brockman is also an occupational name. Brockmans may have included families which specialized in wild brock (horse), and in breaking (Middle English: broke and broken, to break and to have broken) brock horses.