You often see written that quarks cannot exist by themselves and only exist in mesons (2-quark combos) or baryons (3-quark combos). Yet you also see the top quark referred to as existing by itself. Which is right?
What makes the top quark different from the other quarks is its extremely short lifetime. The typical timescale for the strong interaction is 10-23 sec, i.e. about the time it takes light to travel 1 fermi. This represents the time required for quarks to get together and assemble themselves into a hadron. However the lifetime for a top quark is much less than this, roughly 5 x 10-25 sec.
The reason is because the top quark's mass is so much greater than that of the other quarks. The weak interaction causes quarks to beta decay. For example the charmed quark beta decays into a strange one: c → s + W followed by W → e + ν. And weak decays usually proceed at a slow pace because there's not enough energy available to create a real W. As a consequence the W in the reaction is far off the mass shell.
But the mass of the top quark, 173 GeV, provides plenty energy, and when it beta decays into a bottom quark a real W is created. Thus we never observe mesons that contain top quarks, such as "toponium", we always observe the decay of a top quark by itself.