AI bots (specifically RAI) are building Bulldogs from the level one vehicle plant without first building the prerequisite level two constructor.
On a related topic, the Bulldog does very well against Reapers, Golies and Indians, annoyingly well, really. In fact, with equivalent numbers based on either metal cost or build time, the Bulldogs always seem to come out on top.
It's strange that the Core wouldn't have a capable anti-tank tank. The Sumo can successfully fill the role but wreckage can sometimes block the shots from Sumos while Bulldogs can still score hits. A couple of Dominators can fix that so I guess it all balances out.