I. Leaked names:

Sniffer

Rascal

Tuff Golem

II. Hints in the video:

1. All of the chest in the underwater ruins are filled with eggs

2. Something spied on me in the mines

3. My hats keep moving around


III. Matching the leaked names to the trailer:

Rascal means to steal and mischief so moving hats could mean its a mob that steals stuff like armour and tools. judging by how the trailer is its likely its some ghost or undead mob.

Tuff golem would obviously be found in underground if it spawns naturally and since its made out of a block that is found in the mines which is a word used for a mineshaft or a underground area.

That leaves the underwater chest eggs with the Sniffer. The Sniffer could be a proper aquatic hostile mob that guards its eggs in shipwrecks and in underwater ruins. Although sniffing may make no sense underwater it could be a reference to "Sharks sniff blood" maybe the Sniffer can detect mobs with low hp with its nose like a warden and attack the mob referencing the idea of sharks smelling blood.

This is what I think the mobs would be if the leaks are right.