That is what I’m looking for. Firstly, I congratulate you on using them as proper draft animals. Still, to your original question though; the god’s honest truth is I can’t see a reason why they wouldn’t be taxed. Government needs to raise money, so things get taxed; property of all kind including horses. Frankly, it makes more sense to me for a government to raise money taxing a horse than someone’s labour, but that is just me. However, it seems that one’s used exclusively for labour/farming are exempted from that tax, at least according to this link I found:

https://www.cga.ct.gov/2010/rpt/2010-R-0473.htm#:~:text=The%20New%20England%20states%2C%20including,assess%20horses%20for%20property%20taxes.&text=Connecticut%20law%20treats%20horses%20as,of%20their%20fair%20market%20value.

So if they are not used exclusively or even primarily for farming then I can certainly see how they constitute more as a “luxury”.