First of all, most of this theory is verrrrrry loose. It does not have a strong foundation (I've got a lot going on, so I didn't have a whole lot of time to research). Now, the first thing to get out of the way; these two are not directly related, I believe that they are alternate realities from each other. Don't starve, a post-apocalyptic world, where the player has failed to keep the forces of evil at bay.
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TforToe 💨
And Terraria, a reality where the player is able to keep the evil at bay, defeating the final boss, the Moon Lord, and cleaning the world, making it pure. Don't starve, a post-apocalyptic corruption world was consumed by the corruption, leaving everything, including the underworld, in ruins. Now everything of the past, including the monsters have been killed.
TforToe 💨
Only a few remnants of the past still exist. First of all, the oceans, now corrupted, could no longer support the might of Duke Fishron. As he had previously, Duke Fishron once again evolved to support his surroundings. Overtime he left the ocean, returning once again to land, eventually he reproduced, creating a population of pig hybrids on land.
We've all seen the Don't Starve x Terraria Crossover, but believe it or not, Don't Starve is the sequel to Terraria! Don't Believe me? See the proof below.
(This theory is slightly based off of my previous theory, so I recommend reading that also)