In the critique of pure reason, according to my reading, Kant is positing that propositions of mathematics are true because they can be situated in space and time, i.e, they can be conceived in space and time (geometry and algebra). For example, I can conceive of a triangle in space. Since space and time are pure intuitions, making judgements of triangle is legitimate (for eg sum of its angles are 180 degrees).
Now, what if I posit a unicorn? I can definitely conceive of a unicorn in space, just like I can do with a triangle. Someone suggested to me that you need empirical data like color - but then you need empirical data for triangle as well (dots and lines and its colour or width) - but this doesn't look right to me either.
Some would also suggest that I need a concept of a horse with a horn, but all these things separately I can conceive of without any experience. I can literally start from a shape akin to triangle and points in spaces and create a unicorn in my head, just like I can create any shape in space and any particular kind of movement in time.
So will Kant admit that either (1) Triangle must empirically exist for our mathematics to be legitimate or (2) It is legitimate to have a discourse about unicorn - perhaps it is just useless to do that - and the same would stand true for 'God' existing in space and time.