Descartes actually addresses something quite close to your objection.
In Meditation 3 where Descartes makes this argument, he suggests that ideas can have three sources:
- innate - we are born with
- adventitious - come to us from outside (as for example our senses).
- imagined - made by fusing different ideas together
Descartes deals specifically at one point with how we can handle animals that we have never seen. His answer is that we do so by taking adventitious ideas and mixing them together. In the case of a unicorn, we are mixing the idea of a horse with some other ideas we've got like horns.
In your question, you suggest, couldn't we have a perfect unicorn. To answer this, we need to think about what perfect means. In your question, I am not entirely sure I grasp its meaning, but I can imagine you mean one of two things:
- Perfect-1 = most desirable to me.
- Perfect-2 = best thing.
Perfect-1 isn't much of a useful criterion and does not at all seem to be what Descartes has in mind. Perfect-2 appears more useful as a criterion, but it fails to do any good work, because we just punt from "perfect" to "best".
Descartes has something else in mind by perfect than either of these in his "idea of perfection." He means roughly an idea that is unlimited and has what we call "all the perfections." But we actually don't need to completely solve what it is.
We can return to the three types of ideas that Descartes thinks we can have and how we can use these to understand things. Descartes suggests that we understand like this:
- God from the idea of perfection
- Angels from mixing our idea of God and our idea of ourselves
- Humans from our experience of ourselves
- Ourselves from our experience of ourselves
- Animals from our experience of ourselves and our experience of things
- Things from our experience of things
For all of these except God, Descartes thinks we can explain them by what we experience regularly. And that all of them are things we receive in flawed and imperfect ways. (This argument may seem quite dubious to us, but Descartes is less a pure product of modern foundationalism and more a product of medieval scholasticism).
What Descartes finds impressive and uses a proof for God is a type of cosmological proof built on the existence in him of an idea of perfection. This idea stands in contrast to the ideas of himself and animals and things that he can get from experience. First, it contrasts with them in that these ideas are all received imperfectly from the dubious senses. Second, it contrasts with them in having no flaws or gaps in terms of its object. I can have mistaken ideas about cows or myself (see Med. 1 and 2), but I can't on his view have a mistaken idea of perfection itself.
From this, he argues that this idea cannot have come from anything but perfection itself, ergo perfection itself exists.
Why can't this be the unicorn? Well, unicorns are pretty limited things. We see them as occupying a particular place and time and being, well, roughly like horses but different. And that means we can easily see how we combine several adventitious ideas. Descartes's specific example is a chimera (if memory serves) and how it mixes lion and another animal. The same would work here.