BigBill said:It is the "Creature From the Green Lagoon"!
Just a garden slug. However one that size, having both sexes inside of one animal can produce baby slugs at an alarming rate. Where there is one, there are bound to be more. It is not like they can crawl for miles. A control program needs to start quickly!
I agree @BigBill
I would definitely want to control.
A link to some organic control: (I tried to post a link, but as a new member, I'm not allowed)l Many gardeners talk about using a shallow bowl of beer but there are other natural methods as well. (I haven't had a problem with slugs since I've moved to Texas but in California I sometimes used Corry's.) I do see different species of snails than I did in California. I just use a little salt. Not much so I don't upset the soil.
You didn't say what you have planted in your garden, but As @plantladylin
's link says, "slugs can feed on a wide variety of vegetables and herbs, including flowers such as petunias, chrysanthemums, daisies, lobelia, lilies, daffodils, narcissus, gentians, primroses, tuberous begonias, hollyhocks, irises, and fruits such as strawberries. They also feed on carrots, peas, apples, and cabbage that are offered as a sole food source."
Hope this helps.