Building a random encounter table, suggestions?

[Kranford]

so I'm working on some potential encounters through a toxic swamp that's supposed to be teeming with undead, twisted creatures, and other horrible monstrosities. It's meant for folks between levels 12 and 14, and I was wondering if you folks had any comments for my list of beasties. Some encounters run the gamut from easy as pie to a potential boss fight.
They roll this encounter table each time they enter a clearing in the swamp, and under ideal circumstances, will wind up rolling on this table about 13 times or so.

1-3- Advanced Giant Nuckelavee or the Froghemoth
4-5- 1d2 Dark Trees or 1d3 shambling mounds
6-7- 1d2 Quth-Maren (3e Fiend Folio) and 1d6+3 dread warriors (Monsters of Faerun/Lost Empires of Faerun) or 1d2 fleshwarps (Ghonhatines or Irnakurses)
8-13- Nothing
14-15- Pack of Carrion Hounds (1d6+4 plus 1 10-HD pack leader, think rotting wolves with a writhing mass of maggots, maximum EL for this is 11) or 2d4 advanced ghoul dire wolves
16-18- Colony of Fiendish Giant Crocodiles (1d6+4) or Zombie Horde
19-20- Fiendish 9-headed Pyrohydra or 1 Night Twist (3e Monster Manual III)

I'm also doing some figuring on how to have the Nuckelavee show up more often, and any help figuring that would be appreciated. Thanks in advance! :)

[Gariot]

Put the Nuckelavee a second time between 8-10 and 11-13 is still nothing. More chances for the Nuckelavee :D

[Kranford]

Thanks for the tip. The table has been altered a bit now.