A stallion is a male horse that hasn't been castrated. Which is also what an "armăsar" is. From what I've noticed, both in English and Romanian, in casual speech, people tend to use it as "a beautiful horse".
I agree, you'd think the comparison would be between a stallion and a mare, or a stallion and a gelding. I guess that's too much vocab, not that I can see why we need to know the word for a stallion at this basic stage!