I was wandering around the particle date group page for meson and couldn't find a meson for top-bottom, which from symmetry you would expect.
Q1: Is this because it hasn't been found? Q2: There is an underlying reason why it can't exist?
The top quark is so massive relative it's partners that it decays on time scales faster than the hadronization time scale.
Look at the tables again...you don't find any top quark mesons. Instead the top--uniquely among the quarks--has decay modes. (PDF link)