Polygons are shapes made of non-intersecting straight lines and named for the number of corners they have (either in Latin or in Greek, or some mix between the two for whatever reason). Three corners is a tri-angle (lit.: three angles), four a quadri-lateral (lit.: four sides), five is penta-gon (lit.: five corners), six a hexa-gon, seven hepta-gon, eight octagon and ao on.

The joke is that they're treating "hex-bear" as an animal in a line of polygonal creatures, and extending that line in both directions with pent- and hept- animals, such as herpetis (presumably some form of reptile or amphibian, given that herpetology concerns itself with the study of those creatures (though ornithology is traditionally considered a separate field despite birds being related to reptiles (which means that chickens are really just the modern descendants of dinosaurs and I think that's awesome))).