A skeleton can be seen outside of Godzilla's underwater temple. This skeleton bears a body shape resembling Anguirus, a dinosaur who served as Godzilla's ally in a few films; the filmmakers have confirmed that it is in fact Anguirus.
According to Godzilla (2014) art book, The Art of Destruction, the Godzilla corpse found in the beginning of that movie would have been discovered frozen somewhere in Siberia instead of the Philippines. It was changed because Man of Steel, a film from the year prior, had a very similar scene of Clark finding the Kryptonian ship frozen in ice. This film appears to rework it, this time King Ghidorah is found frozen in Antarctica.
One of the Titans, Leviathan, is in Loch Ness, implying that it's the origin of the Loch Ness Monster legend. Toho actually nearly made a kaiju film staring Nessie in 1978 in co-production with Hammer Horror and got far enough into production for posters to be made.
Mothra's eye-spots on her wings were designed to resemble Godzilla's eyes in order to create a connection between the two monsters. Michael Dougherty said 'she is his flag bearer and Queen.'
This is the first incarnation of Mothra that is not a literal divine creature. However, she is still worshiped as a deity by a primitive culture. This incarnation of Mothra to have long forelegs resembles Mothra's pre-production designs by Toho.