Gronkats were animals kept as pets by Space Pirates along with Olbaps. They were all force-terminated after they suffered negative effects from Phazon Fever. Since these creatures have not been actually encountered, nothing else is known about them. The only mention of them in the entire Metroid series is in a scan in the Central Dynamo room in Metroid Prime. Unspecified Pirate pets were referenced in Another Side Story.
"The keeping of pets has been suspended until further notice. Domestic beasts have been deemed highly susceptible to Phazon madness, and are thereby a hazard to personnel. All Gronkats and Olbaps must be disposed of immediately. Failure to comply will result in the loss of pay for a cycle and reduced rations."