I think people are being a bit rough here.
Where she gets her money is no business of anyone else's. This is a horse forum - not a "how to live your life according to me" forum. People might throw stones at me, but I am on a government allowance (for students - pretty much all students get this in my country) and I have a horse. I work part-time as well, but, unlike many people on here, its not that easy to get regular hours sometimes. It's easy to criticise someone's financial position when you're not in it. It blows my mind how people think they should criticise someone else's choices because they're not like theirs. I know if I had a child I'd never want to work 10 hours a day. In fact many people choose not to work full time and I think that is perfectly fine. Its only really recently that full time hours has become expected of everyone. Money isn't always worth time.
To me it doesn't sound like she can't afford the horse, more like she can't afford to be spending money for something that isn't needed. The OP also sounds like she needs to grow up a bit too. You're an adult, a mother, don't let someone push you around. Owning a horse is a responsibility, much like having a child, and once you have that responsibility you have to step up. If you're experienced enough to own a horse then you know what that horse needs, and if its suitable for you regardless of what your mother says.
Work out the reasonable money owed and move your horse immediately, upon payment of your reasonable debt, then decide what you want.
Its not really a problem unless you're making it one.
I also think that if you don't believe that money is worth the time, and instead are going to get benefits when you *could* be working and *could* be earning that money instead of the people who really do need it getting it, for something unnecessary like a horse, that's morally wrong. I know 100 percent that if I was in such a position where I needed to accept government assistance, I would sell my horse. It would be hard, but if I can't afford to live my life with my own hard earned money than I shouldn't have a horse.
I kind of agree that money isn't always worth the time. That can be true. But not often. If it's few dollars per hour of grueling, backbreaking work - than yes, it's not worth it. But even a minimum wage job is worth the money. Everybody has to work for their money. If you don't think you would have enough time for our daughter if you worked full time, then would you really have time for a horse, too? TONS of people work full time with kids. And tons of people work part time, and still make it work. But if you want extra luxury things like a horse, you should get off your butt and work for it. If you're going to take unneeded government assistance when you could be working, you shouldn't have a horse. (This talking about OPs situation. Yours sounds different.) We all work for our money, save for people who legitimately can't. That system should not be taken advantage of.
We of course can all agree to disagree. She did ask our opinions on what we should do. And we, for the most part, are giving it (Sell the horse for X reasons). I can see where you're coming from... I just happen to have a different view on it than you. Sorry if I came off snarky :p