1 And him beside marched amorous Desire,
him beside > [beside him]
2 Who seemed of riper years than the other swain,
swain > young man; fellow
3 Yet was that other swain this elder's sire,
sire > father (i.e. Fancy is the father of Desire)
4 And gave him being, common to them twain:
them twain > the two of them
5 His garment was disguised very vain,
disguised > {To "disguise" is to alter the fashion or guise of clothing, and to dress in an unusual, fantastic, ostentatious, or inappropriate manner. Here, "disguised" might be taken to mean "got up"}