If the cost price of 12 pens is equal to the selling price of 16 pens,

Let the $\mathrm{CP}$ of each pen be Rs $x$.

CP of 16 pens $=$ Rs $16 x$

SP of 16 pens $=$ CP of 12 pens $=$ Rs $12 x$

i.e., $\mathrm{CP}>\mathrm{SP}$

Now, loss $=\mathrm{CP}-\mathrm{SP}$

$=(16 x-12 x)$

$=R s .4 x$

$\therefore$ Loss percentage $=\frac{\text { loss }}{\mathrm{CP}} \times 100$

$=\frac{4 x}{16 x} \times 100$

$=25 \%$