Fel elementals, also known as "dog-sized tentacle things", were a type of monster found in Ginaron. It is unclear if they were created by the Ytarans as part of their experiments into planar travel or found somewhere in the Shadowfel.
Mechanically, they had DR5/good.
10 Fire resist
10 Cold resist
Fast heal 2
Every time someone hits them in melee you get 1d6 backlash negative energy damage. You can get around that if you attack with reach weapons and/or ranged weapons.
They don't suffer electricity damage but every time they got hit by electricity they have to make a Fortitude save or get stunned for 1 round. While stunned they seem to gooify and suffer from all the penalties you get for being stunned.