鲤 is a bound form (b.f.) meaning 'carp'. It rarely appears alone in modern Chinese; instead, it forms compounds, most commonly 鲤鱼 (lǐyú, 'carp'). You'll also see it in 锦鲤 (jǐnlǐ, 'koi') and scientific terms like 鲤科 (lǐkē, 'Cyprinidae family'). When referring to the fish itself, always use the compound 鲤鱼.