Hi Vincent,
When I come across the name of an animal or a plant in English, dictionaries are usually not of great help.
In such cases, I usually look them up in Wikipedia. For instance,
here's the page on "Stingray" and
here's the one on "Manta Ray."
If you're lucky (as I am), in the bottom left-hand side of the page you'll find a link to a similar page in your language, which will enable you to see what they are called in your own language.
Another tool you might find useful is "Google Images." Here are
some pictures of Manta Rays, and
here some of Stingrays.
