There is the theoretical dibrayon-
http://en.wikipedia.org/wiki/Dibaryon
and the theoretical pentaquark-
http://en.wikipedia.org/wiki/Pentaquark
From 'Neutron Star Interiors and the Equation of State of Superdense Matter' page 9-
3.3 H-matter and exotic baryons
A novel particle that could be of relevance for the composition of neutron star matter is the H-dibaryon H=([ud][ds][su])), a doubly strange six-quark composite with spin and isospin zero, and baryon number two. Since its first prediction in 1977, the H-dibaryon has been the subject of many theoretical and experimental studies as a possible candidate for a strongly bound exotic state. In neutron star matter, which may contain a significant fraction of
Λ hyperons, the
Λ’s could combine to form H-dibaryons, which could give way to the formation of H-dibaryon matter at densities somewhere above ∼ 4
n0. If formed in neutron stars, however, H-matter appears to unstable against compression which could trigger the conversion of neutron stars into hypothetical strange stars.
Another particle, referred to as exotic baryon, of potential relevance for neutron stars, could be the pentaquark, \Theta^+([ud]^2\bar{s}), with a predicted mass of 1540 MeV. The pentaquark, which carries baryon number one, is a hypothetical subatomic particle consisting of a group of four quarks and one anti-quark (compared to three quarks in normal baryons and two in mesons), bound by the strong color-spin correlation force (attraction between quarks in the color \overline{\textbf{3}}_c channel) that drives color superconductivity. The pentaquark decays according to \Theta^+(1540)\rightarrow K^+[\bar{s}u]+n[udd] and thus has the same quantum numbers as the K^+n. The associated reaction in chemically equilibrated matter would imply \mu^{\Theta^+}=\mu^{K^+}+\mu^n.
