The exact same sentence but with boys instead of animals was accepted with "the" in it. Without context I can't tell if specific or general animals are here so both translations should be accepted.
I believe the Czech here is a general statement. If a statement with 'the' in it were meant, then the Czech would have added the appropriate demonstrative adjective.
So the solution is not to wrongly accept a translation with 'the' here, but rather to correct the previous sentence you refer to, if it is in fact wrong.
There are two different types of adjectives in Czech.
'Hard' adjectives end with -ý (masc sing nom). Those adjectives have different plural nominative endings according to gender: -í for masc animate, -é for masc inanimate and fem, -á for neuter.
'Soft' adjectives end with -í and they have only one plural nominative ending -í, regardless of gender.
If the definitive article is "implied" in most cases why could not this be translated as "The animals are strange" which was marked wrong.
No it is not always implied. It is only implied if it is clear which specific objects you mean. I can't imagine this (Czech) sentence to be about some particular specific animals.