Today we are given a story that in the end leaves us with more questions than answers (that was intentional). The old man wasn't clear on what he saw and didn't give enough detail to readily figure out what it is but with some investigation and tracking our party would learn that the huge creature being fed by the bullywugs indeed a froghemoth. The froghemoth is a rather interesting creature because of the adventure and investigations it could bring to your party. The froghemoth leaves open a few possibilities that would be so fun to entertain as a dungeon master / game master and would also be really fun for the party to RP. First off the froghemoth is a creature driven purely by hunger, so according to Volo's it can be tamed rather easily if offered food. This does mean however, if the party manages to tame the froghemoth they would have to be very careful to never run out of food otherwise this monster would throw a tantrum bigger than the tantrums  of my children when I say "no you can not jump off the top bunk onto hardwood floor" (and just think the froghemoth isn't even as smart as a young child) and with a CR 10 this tantrum could very easily be deadly to a party. This would be a fun situation for both sides of the table. My second idea came from Volo's, it says that the froghemoth will lay eggs on its own and cares nothing for the eggs it lays so I thought..... LET THE PARTY FIND AN EGG!! We all know players go gaga over eggs. Personally I would make it possible (big emphasis on POSSIBLE) not necessarily easy, for the party to turn big boy into a pet after hatching. Volo's states that they grow to full size in a matter of months but I would just determine how string you are okay with it getting (if they keep it as a pet) and just let it grow to that point over the course of the campaign. Like I said some really fun stories could be made with this guy for both DM / GM and players alike! Thank you for listening and be safe adventurer!