1 EV-9D9: Ah, good. New acquisitions. You are a protocol droid, are you not?
2 C-3PO: Yes, but... Why is that upside down droid being branded on the feet?
2 Gonk droid: Gonk! Gooonk!!
2 SFX: Sizzle!
3 EV-9D9: It’s to show how sadistically cruel I am.
2 SFX: Sizzle!
4 C-PO: It’s a droid. Did you give it the ability to feel pain too??
4 EV-9D9: That’s how cruel I am.

Canonically, the droid being branded in Jabba's palace is an EG-6 power droid, but it's commonly known as a Gonk droid, because it's one model of the GNK series of droids, known for their "Gonk!" vocalisations.

We've mused about why a droid might be programmed to feel pain before. But why would a Gonk droid, which is basically a big battery on legs, have pain circuits? Unless they were installed by the torturer?

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Reader Steve M. writes:

They were. In Tales From Jabba's Palace, there's a short story told from the perspective of the sadistic droid. It's got some aftermarket addition that lets it sense droid distress over short-range radio transmissions, or something, and gets off on it. The torturer installs "pain buttons" that transmit that distress in all the droids in Jabba's palace, and then tortures them to savour that distress.

This all became non-canon after Disney bought the franchise and tried to pretend 40 years of tie-in novels didn't exist.