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It's very unlikely that anything as big as Mercury exists in the Kuiper Belt because it would be eight times bigger than Triton, Xena and Pluto (Triton is Neptune's largest moon, but it's a captured KBO, slightly larger than Xena and Pluto). There certainly aren't going to be 'a whole bunch' of them that all happen to be at the celestial poles at this point in their super-inclined orbits. The fact that the three largest KBOs found even after most of the sky has been surveyed are all about the same size actually suggests that the biggest that objects formed in that region could grow to was about that size, just like there are physical reasons why Ganymede and Titan are as big as icy moons can grow.
Read Michael Brown's explanation that Haku pointed a link to. It would be a nonsense to classify Pluto and Xena, let alone dozens of smaller objects, as planets when they're just the largest members of a population of small bodies. Mercury is much bigger and it's not much smaller than Mars. It isn't part of a population of small objects like they are, it's the smallest of the big objects. Mercury is 25 times more massive than Xena and Pluto. It's several times more massive than the entire Kuiper Belt. It's the obvious place to draw the line. If something as massive as Mercury is found in Kuiper Belt, it should be a planet, but I'd bet money that one won't be. What's your problem?
I'd already discussed the possibility of finding big, planet-sized objects beyond the Kuiper Belt. They may exist and if they do they should be recognised as planets.
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