So apparently the Dungeon Rover Archetype for the Ranger, gets the option of a Giant Slug at level 4.
A quick look in the Beastiary reveals the Giant Slug to be a CR 8 Huge sized monstrosity, sporting such things as 27 Strength, 16 Natural armor and a 10d6 Acid Spit. Just to name a few.
Surely this is some sort of error?
Most animal companions have "special" stats unique to them. Check the animal companion page, there is certainly a "lesser" version that must be used there.
Slug, Giant
Starting Statistics
Size Medium; Speed 20 ft.; AC +4 natural armor; Attack tongue (1d4 plus 1 acid); Ability Scores Str 13, Dex 8, Con 13, Int —, Wis 10, Cha 1; Special Attacks spit acid (ranged touch attack, 30 foot range, 1d6 acid damage); Special Qualities blindsense 30 ft., DR 5/slashing or piercing, susceptible to salt; CMD can’t be tripped.
7th-Level Advancement
Size Large; AC +2 natural armor; Attack tongue (1d6 plus 1d2 acid); Ability Scores Str +2, Dex –2, Con +2; Special Attacks spit acid (ranged touch attack, 30-foot range, 1d8 acid damage); Special Qualities blindsight 30 ft.
The slug is in the same book as the insect companions I believe. I cannot remember which though UM maybe?
http://www.d20pfsrd.com/classes/core-classes/druid/animal-companions#TOC-Sl ug-Giant
Fixed for you
Seems a pretty decent animal companion to me -- blindsight and acid spit, with a (weird) DR and some decent natural armor.
Just stay away from salt.