[pygy_]

> it must be either san-serif or serif, right?

Not necessarily. This font is a monospaced hybrid of slab serif (fat serifs, regular line width) and sans serif.

See here for details: http://en.wikipedia.org/wiki/Serif

[illicium]

To be fair, most monospaced typefaces are at least to some degree slab serifs to visually compensate for the extra horizontal space (eg. letters like "l" and "i")