Typically hybridization can only occur between closely related species within the same genus or, in some cases, related genera. Black pacu (Colossoma macroponum) and red-belly pacu (Piaractus brachypomus, formerly Colossoma bidens) are separate species within related genera that, I believe, have or may hybridize. Other than that, there aren't many closely-related species I can think of that might be genetically similar enough to allow for successful (if you can call any hybridization a "success") hybridization. More importantly, why?