I know it has been suggested before but allow meto really flesh it out!
The thief would be represented as a raccoon as I think the animal suits. However, a cameleon is also a good animal representative of the class.
Dexterity based combatant: the class would be what the Archer evasion melée build was like. They use daggers and bucklers.
Manipulates status effects: specifically poison, the thief utilizes stuns and various status effects to get the upper hand on the enemies.
Stealing: the thief gets a marginal increase on drop rates (5%) passively and activating a skill, can gain a percentage of extra gold.
Stealth: can go invisible for a short period of time to evade enemies which can also initiate specific skills that deal extra damage if utilized while in stealth.
Dual Wielding (use two daggers)
Just food for thought!