and if you think about it there really aren' t any words of latin derivation in which e is pronounced ai
The "betta" in Betta splendens isn't from Latin, it's from Thai or Malay I believe. Splendens is from latin.
It's somewhat similar to a Hindi/Urdu word, but whether they come from the same root is beyond me (SE Asian languages does have major Subcontinental influence on it, so I wouldn't be suprirsed).