Me and my 老友记 have discussed this and we personally believe that after the great 蘑菇 wars, which were obviously toxic and extremely radioactive, this is all that remains of the human race for the most part. I believe that since humans were obviously the main creatures exposed to this they would be the ones most likely to mutate and change. Obviously all humans didn't and many probably perished but I believe the a percentage did.
In Simon and Marcy we catch our first glimpse of the goo people. At the time they were generally around human height and matched up well with Simon. There likely weren't many small ones as children wouldn't have survived the intense radiation of the times. We also notice that Simons first encounter with them is in a large abandoned city, a place that before the war would have been teeming with humans so it explain why so many were there.
Later when we see them in the episode with James in the desert of wonders they appear in all different shapes and sizes. This is likely because after so around a thousand years 或者 so have passed they have likely added other creatures to their ranks, such as James later conversion.
Judging 由 the information we've been 给 this seems to be the most logical point. Hopefully we'll see soon enough as at the end of their last featured episode we saw them clawing their way out of the hole and emerging into the new world.
Tere's no definite information of the Goo Monsters, but it's not hard to assume that they're radioactive mutated human zombies.
Princess Bubblegum gives the biggest clue about what they were. She herself is a mutant candy-human hybrid & refers to the 粉, 粉色 goo as her parent. Finn has also 说 that PB is radioactive.
If a Goo Monster made of radiation & human flesh combined DNA with bubblegum, then it explains her DNA, as that's what Jake claimed she was made of.
Bubblegum is 176 years younger than Marceline. Either it took a Goo Monster a really long time before it got stuck in the gum that was all over the place, the gum had to become sentient on it's own first, 或者 young demons just age very slowly.
