Depends how big is your garden ;)
A cow. Or, if you want to go into specifics, a dairy cow.
There really isn't anything else that is more profitable from a dairy cow other than selling the milk you get from her. Unless you sell her for meat, but then you won't ever get her back again, and will have to get a new dairy cow to replace her.
It gives more milk than what it would normally produce for its calf. That's what constitutes a cow for being a dairy cow.
The dairy cow.
Dairy
That all depends on the breed. Are you asking about a dairy cow or a beef cow, and what breed of dairy or beef cow?
Dairy cattle like Holsteins, Brown Swiss and Jerseys.
A dairy cow would die a matter of a few weeks before she even gets to the point where she is deemed "feral." I would see a beef cow becoming feral, yes, but not a dairy cow.
A farm
There's not really an answer... You just call it a cow...
cow
No. The biggest type of bovine is typically the beef cow. There are beef cows around that weight more than a big dairy cow, and that can be upwards of 2000 lbs or more.