Pilgrim slipped aside to look, and saw how hideously ill-favoured the fiend king was:

生得 takes 得 as the light de of how a thing turned out. 好不兇醜 looks like a negative and is not: 好不 in front of a description intensifies it, so this means thoroughly hideous, not "not hideous". The construction still trips up learners, and Ming Chinese uses it constantly. The tag also opens a set-piece, so everything that follows is verse.

A single horn, jagged and uneven; two eyes glittering bright.

參差 is one of those pairs where neither character takes the reading you would expect: it is cēn cī, uneven and out of true, and not cān or chà. The two syllables rhyme with each other and mean nothing apart. And the single horn is not decoration — it is the fiend's name and his entire nature.

On the crown the coarse hide bulges; at the root of the ear black flesh gleams.

The portrait works downward from the crown of the head, which is how these descriptive poems are always built. You are being walked over a face.

The tongue, when it lengthens, stirs at the nose; the mouth is wide and the slab-teeth yellow.

長 here is the adjective cháng, long. 版牙 are broad flat teeth, the grinders of an animal that eats grass — and that is the first hint of what is really being described. Look at the mouth, not the horn.

Hide blue as indigo; sinews knotted and hard as steel.

靛 is indigo dye, so the hide is that deep blue-black. 筋攣 is sinew drawn up into knots. The colour is worth filing away; it matters later in the arc.

Set him beside a rhinoceros — his horn could not light the water; liken him to a bull — he ploughs no waste ground.

照水 points at a story every educated Ming reader knew: Wen Qiao, in the Jin histories, burned a rhinoceros horn at Niuzhu to light up the water and see what was living underneath it. Rhinoceros horn was supposed to show you hidden things. 牯 is a bull. The couplet defines the creature entirely by two uses he does not have: a horn that shows nothing, a bull's frame that ploughs nothing.

Nothing at all of the ox that pants at the moon or ploughs among the clouds — instead, a strength that bullies Heaven and shakes the earth.

喘月 is another one every reader would catch — 吳牛喘月, the ox of Wu that pants at the moon because it has been so scorched by the sun that it mistakes the moon for it. It is still current Chinese for flinching at a thing because something like it hurt you once. 犁雲 is ploughing among cloud. 倒有 takes 倒 as dào, the "on the contrary" sense, not dǎo, to topple. He has an ox's body and none of an ox's usefulness.

Two hands of scorched sinew, indigo-dark; in fighting majesty he holds the steel-tipped spear straight before him.

雄威 is martial authority, the bearing of a fighting man. The description ends on the weapon, which is where the next passage begins.

Look closely on so fierce a shape: he is not wrongly styled the Si King.

不枉 is "not for nothing, not undeservedly"; 名稱 takes 稱 as chēng. The poem signs off by confirming the name the mountain god gave five passages back. The reader is being told: the god was not exaggerating.