Large Grasshoppers? Woodpeckers?
JoJo

Mice? That's what happened to mine, and it was on a 2nd floor balcony!

Nancy

It does look tasty. Juicy once past the prickles. A cucumber in another life.

I have had several succulents and bonsai trees destroyed by squirrels (including wood tables, chairs, cushions, pots). But not sure what caused this to your cactus. You should get some toxic plants :)

I have a nice sized euphorbia right behing it. (You can kinda see its green self in the back of the pic)
That also had some bites taken out of it, but not nearly as severe as this one. Maybe i should search around a bit to find it's remains...

Probably not a solution for most people, but I built two cages (6x6x2 feet PVC pipes), draped one in a 30% shade cloth and the other in bird netting (0% shade) and put them on my garage roof. It's a win win for me because they are protected from both varmints and too much sun. And they get full sun from morning to evening (which I assume is the ultimate, but I could be wrong, haha). When I kept my plants in my yard they would be in shade half the day, then get scorched for a few hours.