Hi Marco,
According to Merriam-Webster (the top dictionary of American English), a "pocillo" is a "small coffee cup." Based on that picture, I wouldn't call it a "coffee" cup though...
Google translated it as "well." That is clearly not a well (un pozo).
Personally, looking at the picture alone, I'd simply call it a "sauce bowl." Maybe someone who knows bowls and plates better than I do can provide you with a better answer =\
I hope this was at least somewhat helpful.