What did Team Rocket use to evolve Rhyhorn?
No reliable site lists what that syringe is (or could be) and it never appeared again. Misty asumes that an other of these syringes was used to evolve a gyarados (seen earlier).

Team Rocket tried to create ways to force an evolution (like the experiments at the lake of rage), so it's most likely another experiment to force the evolution of a pokémon and to make it stronger.