Hope a necro post will be okay, since this popped up from a search and feels most appropriate to add to rather than starting from scratch.
Problem is, Rhyncho is just an obstacle and has very little incentive to be a target itself. How about making just the Rhynchos shared loot? Few solo hunters can tackle them anyway, but the chance of looting something good from a high-hp mob should attract teams. Shared loot takes care of the hassles of making people get organized, but doesn't exclude soloists. Okay, you don't get your name posted on EL when you swirl, but that's the only disadvantage I can think of. No mission on their heads means no arguments over points either. In the shadow of the action, folks can hunt their Skull Birds.