The ochre jelly, j, is an amoeboid monster that appears in NetHack. Like other jellies, they have a dangerous passive attack - the ochre jelly's acidic passive can corrode iron or copper objects. Unlike other jellies, however, they are the only non-sessile member of the monster class, and can engulf players to cover them with acidic slime; the engulfing attack does not corrode objects, despite being acidic.
Ochre jellies are the second quest monster encountered on the Knight quest, and have a 14% chance to be generated; several ochre jellies are guaranteed to appear in the floors beyond the home level.
Ochre jellies are quite slow, so you can easily run away if you aren't prepared to handle them. Their passive acid attack does a fixed 3d6 damage roll, as opposed to the (XL + 1)d6 of spotted and blue jellies, making their passive much less dangerous in comparison. Like most solid engulfing monsters, a wand of digging or spell of dig will immediately free you and reduce their HP to 1.
Players keeping mind flayer pets will want to keep them away from ochre jellies, whose acidic passive will trigger with each tentacle attack; it may be worth giving them yellow dragon scales/scale mail or a spare alchemy smock for acid resistance.
In SLASH'EM, the engulfing attack can now corrode objects in open inventory, including your alignment key.