Open your inventory, move the Cabbage to a spot in your hotbar, and close the inventory. Once you're out of your inventory, select it from your hotbar, and act like you're using a weapon, as if you're swinging a sword. That should summon it. Once it's summoned, you can move it off your hotbar.
I doublechecked and tried all my pets and they work. One thing that might be messing you up is when you swing to activate the pet, it can stand behind you and you can't see it. Actually you are kind of standing ON it. So swing, then move out of the way and walk forward to see if your pet was covered up by your player. If you swing again, you'll unsummon the pet. So try it a couple times.
Actually I have a cabbage as well and it has never worked for me either. I have multiple other pets, and I know how to get those to work, but the guinea pig just won't show up. I think this is just a bug that will need to be fixed in a future update. Sorry!
Hmm, maybe it's just the little hamster Cabbage that doesn't work. I don't have that pet on my this playthrough so can't confirm it works. All my other pets did.