Talk to your accountant, but I can give you a quick overview. You will pay taxes on the sales of foals whether you are running a business or NOT! It is either hobby income or business income. The biggest difference (besides the actual IRS tax forms you use) will be on what you can deduct. As a business, you must have intent to earn a PROFIT (not just income). One of the IRS “safe harbors” is showing profit in 2 of 7 years - it is not required, but it does show you have a viable business. As a business you can write off many expenses (including depreciation of breeding stock, fencing, etc) that you can’t write off as a hobby.
For profit means you have a realistic goal of having income greater then expenses - not just that you are selling foals.
And, although this is much more complicated then I can discuss here, when running a hobby, you also have to consider AMT (alternative minimum taxes) - you may not be able to write off ANY expenses depending on this calculation. You may just have to declare the income from sales with no offsets.
OTOH - if you are not running a genuine business, you really can’t justify it as a business. And odds are, with 2 broodmares, you aren’t going to be able to show profit. You need to pencil out a business plan with realistic budget (including not just direct costs like feed and vet bills, but allocated cost such as depreciation of barn, fencing, etc) and see if you have a viable business. If you are audited, this is something you will need.
There are also State requirements to consider - I am in California, and as a business that SELLS, I have to get a resale license and file sales tax reports - and yes, horses are subject to sales tax which eats another 8% out of every in-state sale I make. This is a Cali requirement - it varies depending on where you live.
And you can have a job - but if you start paying employees to do all or part of your work, then there is even more documentation required to show you are an active participant (unless you incorporate and are not the “sole proprietor”). Most of us work at least part time outside the home to pay for our own groceries! Until you get into the really big breeders (like Rolling Stone or Hilltop or Iron Springs), you’ll find most breeders working “real jobs” too.
It is a hard business - to be honest, most breeders are underwriting the rest of the horse world - the riders, trainers, vets, etc. If I knew then what I know now…