Russia/Estonia is a lesser-known pairing involving the characters Russia and Estonia, and is loosely based on canonical content. The pairing is sometimes known as RusEst, combining the first three letters of Russia's and Estonia's names.
The majority of fans ship it due to Estonia being the least noticed, or least remarkable of the Baltic trio, and seems to have fewer run-ins with Russia altogether, leading fans to believe that Russia cares more about Estonia then the others.
Another potential making of this couple comes in some episodes in the 5th season of Hetalia:Beautiful World. For instance, in Extra Episode 10: I Was Overwhelmed by Heroines, at the end of the episode, Russia is seen putting his chin on Estonia's head, and both of them are blushing with flowers floating around them.