Cornell Box
One of the rooms on Horizon is modeled after a Cornell Box, which are used to test realistic 3D rendering.
Dolphins
Added in Season 2, the dolphins are one of the three easter eggs required to unlock level 3 of THE FINALS Easter Egg Hunt. They can be activated by 7 players standing on boxes marked with letters on the sunset variation of Horizon.
On April 4th, Rob first hinted at the dolphins by telling hunters "don’t forget to watch the sunset…". The next day, the activation page for level 3 was released. Among other easter eggs, it featured a dolphin made of voxels associated the 2.0.0 update, which suggested the Season 2 arena Horizon.
On April 7th, hunters frösty and RockhoundBlack noticed letters on some boxes on the arena's sunset variation. Collectively these spelled "cetacea", the infraorder of which dolphins belong to. Soon afterwards, hunters tried standing on each box at the same time, which triggered three dolphins to emerge from the water in front of the sunset.