Rocket Grunts were added in v0.4!
They can randomly spawn in any biome at any time of day.
If they catch sight of you, they will walk up to you and release their pokemon for battle. Most Rocket Grunts have Koffings or Bulbasaurs as their pokemon. Their pokemon levels differ at around level 30.
Once defeated, a Rocket Grunt may drop some pokeballs, a rare candy or maybe a Pokedex .
However, a Rocket grunt can be killed, but won't drop anything. And the rocket grunt can deal some damage to you if you're too close up to him.