A slug is a small, slimy creature with antennae, usually considered mildly disgusting by humans. There is no difference between different types of slug (such as banana slugs vs. leopard slugs), except for things that don't effect a stat block, like diet. The exception is Underdark slugs, called Undarslimes, or black slugs (though they actually are mottled gray, to better blend in with the stone). They have a Stealth bonus of +1, or +3 when still, and, should a DM choose to make a character make a check to spot one in their natural habitat, the DC would be unusually high. A snail can use the same stats as a slug, with Quifiaf (KWIF-ee-AF) being Underdark snails (gray shell, same Stealth bonuses). Snails have an AC of 7.

With your DM's permission, you can summon a slug (or a undarslime, snail, or quifiaf) as a familiar using a spell such as find familiar.