This is probably just how you say 'turtles' as in 'all THE turtles' in Hungarian/turtles in general, rather than just some of them. (In French, it'd be 'Les tortues...')
"The turtles are slow animals" is bad English whereas "Teknősök lassú állatok" (without the "a") is bad Hungarian. Sometimes a word for word translation does not work.
Just to clarify: I think 'turtle' can be what Brits. call 'tortoise', whereas a 'turtle' in British English refers to the marine animal.
Why is teknősbéka marked wrong for turtle? It's what my dictionary and all the online dictionaries I use give for turtle... I understand that the béka isn't required but surely it isn't wrong to have it??? And there's no way to report it in either of the exercises where it's come up.