A guest shark is a shark brought into the tank to either supplement or temporarily replace one of the main cast. Guest sharks have been featured throughout the run of Shark Tank though they were far more common in Season Nine where the majority of episodes featured one.
From a numbers perspective, their contributions to the statistics of the show are minimal given that the majority of guests have appeared for only an episode or two and have made very view investments for usually very small amounts. Their numbers are included among the season, category, and lifetimes totals however as they do represent a (minuscule) portion of the deals made within the tank. Also, their deals are frequently made with main cast sharks and to more accurately portray the numbers it is actually easier to have them within the data than to try and exclude them.