Poecilia sp. (Domestic hybrids). These ubiquitous livebearers will breed quite easily when kept in mixed-sex groups; we recommend at least 2-3 females per male to keep them from getting harassed too badly. Mollies are very hardy fish and perfect for beginners.
This is the Gold Dust variety, with jet black tails and golden metallic front halves. They are young adults around 2-3" long.