So yesterday, I found out something about Hutch: his inventory isn't safe from Splumonkeys, not even in Fugu form.
Nothing here made me want to burn their homes more then discovering that!
The way I see it, if Hutch is covered in spikes, he should remain safe from pesky prying hands, but I don't think they need to be damaged by his spikes. Frankly, there are a couple of options here: #1. Spikes=Monkey Repellent: they don't get close to Hutch, and there's nothing to worry about. (This option might be too easy but players will probably love it.)
#2. Monkeys try to steal from Hutch like normal, but fail by being lightly stung at little-to-no damage to themselves (after all, we can't possibly open up that kind of monkey farming method). Each failed attempt slightly lowers the spear's durability (possibly more-so then a retaliation from a normal attack), requiring some attentiveness and the occasional replacement by the player.
#2b. Fugu Hutch only prevents most of their stealing attempts. Each time a Splumonkey tries to steal from Hutch, a roll is thrown, and 9 times out of 10 or so (or whatever percentage you choose), they simply get a handful of spikes. For the remaining percentage though, they will successfully knock loose an item (which they can then take). Hey, you stick a bunch of splumonkeys in a room, and they'll eventually write Shakespeare. But seriously, it's pretty easy to assume that they might not know how to open up a spiky fishball chest. In short, a mighty source of shenanigans.
There's also #3. Take a third option! (And I've got 2 of them!) #3a. Make it so that Music Box Hutch soothes and distracts splumonkeys from their thieving attempts. An expensive option (and perhaps a task in futility since splumonkeys just chase you anyways), but perhaps it will be of some value to those who desperately desire a reprieve.
#3b. Splumonkeys will choose to steal Light Bulbs off of Hutch's lure before trying to take anything else (other then Bananas maybe). This is both good and bad; while each light bulb stolen is simply more one thieving attempt that has been diverted (which is good), it also mean more food for monkey manure (which is bad). It'll also mean that players dependent on light bulbs for fuel should pay attention to Hutch if its their only supply.
Otherwise, if nothing is done, it'll just be Planet of the really obnoxious cave monkey.
Besides... you added Hutch to be the Underground Chester, it'd be a shame if he was subjected to the same troubles Chester had.
tl;dr: Hutch has a spiky fish form, and splumonkeys can still steal from him.