The Coward is a yellow (in both ways) creature that runs like hell at the merest sight of an enemy. The animal is very hard to kill, and your best bet is to corner then spam it.
Its location is roughly here (in the Kelp Forest)
And it looks like this (though you'll probably only see a yellow blur
As for the strange fish, they are in the Abyss, inside the whale room (though not inside the whale itself). If you travel all the way to the left on the ceiling of the room, a gap will appear - it leads to a small tunnel which, in turn, leads to the Easter egg.
: WARNING SPOILERS
Did I say spoilers? SPOILERS XD
Good luck! Hope you enjoy the rest of the game, and have yourself a wonderful Christmas. BTW, next time try a forum search - took me five minutes to compile all the pictures.