梅花鹿聚集吃草

夜晚，四只年轻的梅花鹿聚集吃草。它们红茶色的鹿，有美丽的斑点。它们都耳朵挺立，目不转睛地看我，过了一会儿，它们到森林里，不见了。

夜晚，看到四只年幼的梅花鹿聚集在一起吃草，它们茶红色的鹿身上点缀着美丽的斑点。当我走近，他们都竖起耳朵目不转睛地看着我，过了一会儿，跑进森林里，不见了。
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梅花鹿聚集吃草

夜晚→晚上，四只年轻的→年幼的梅花鹿聚集→聚在一起吃草。它们红茶色的鹿→它们毛色微红，有美丽的斑点。它们都耳朵挺立→它们竖起耳朵，目不转睛地看我→看着我/望着我，→。过了一会儿，它们跑到森林里，不见了。

* general collocation:
   a young person=年轻的人
   a young animal=cub=年幼的动物（即幼兽）

Four of young Sika deer gathered at the meadow to eat grass / eating grass in the night→at night. They were reddish brown and had beautiful spots. They pricked up their ears, gazed→gazing at me, and a while later they disappeared into the forest.

*  ~four of deer:
     do you mean there are four deer eating grass among a group of deer? if so, "four of deer" is ok, but you'd better mention that there are other deer doing sth else. if not, "of" can be cancelled.
   ~gather to do sth:
     do you mean they gather there in order to eat grass (i.e. eating grass is their purpose to gather there)? if so, "gather to do sth" is ok. if not, you just want to describe their status or the accompanying action, use -ing form may be better. "gazing at me" also describe the the accompanying action when they pricked up their ears.

ps I only watch the deer in the zoo. I envy your close observation to them. ^^
 

梅花鹿聚集吃草

夜晚，四只年轻的梅花鹿聚集吃草。它们（是）红茶色的鹿，有美丽的斑点。它们都耳朵挺立，目不转睛地看我，过了一会儿，它们到森林里，不见了。

Four of young Sika deer gathered at the meadow to eat grass in the night. They were reddish brown and had beautiful spots. They pricked up their ears, gazed at me, and a while later they disappeared into the forest.

Congratulations，your article is no problem， only need add “是”.

梅花鹿聚集吃草

夜晚，四只年轻的梅花鹿聚集在一起吃草。它们是红茶色的鹿，有美丽的斑点。它们支楞着耳朵，目不转睛地看我，过了一会儿，它们到森林里去了，不见了。

Four of young Sika deer gathered at the meadow to eat grass in the night. They were reddish brown and had beautiful spots. They pricked up their ears, gazed at me, and a while later they disappeared into the forest.

梅花鹿聚集吃草

夜晚，四只年轻的梅花鹿聚集在一起吃草。它们是红茶棕色的鹿，有美丽的斑点。它们都耳朵挺立，目不转睛地看我，。过了一会儿，它们到消失在森林里，不见了。

Four of young Sika deer gathered at the meadow to eat grass in the night. They were reddish brown and had beautiful spots. They pricked up their ears, gazed at me, and a while later they disappeared into the forest.