[{_id:ObjectId("61142a398f4cfec27b46e89d"),animal:"cat",action:"meow"},{_id:ObjectId("61142a438f4cfec27b46e89e"),animal:"cat",action:"stalk"},{_id:ObjectId("61142a4a8f4cfec27b46e89f"),animal:"cat",action:"claw"},{_id:ObjectId("61142a7e8f4cfec27b46e8a0"),animal:"dog",action:"woof"},{_id:ObjectId("61142a838f4cfec27b46e8a1"),animal:"dog",action:"beg"},{_id:ObjectId("61142a918f4cfec27b46e8a2"),animal:"fish",action:"swim"}]

db.collection.aggregate([{"$group":{"_id":"$animal","actions":{"$push":"$action"}}},{"$project":{"_id":0,"animal":"$_id","actions":1}}])