Go with Data structure

petowner_json.go

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
)

type Pet struct {
	Owner string `json:"owner"`
	Pet   string `json:"pet"`
}

func main() {
	data := bytes.NewBuffer([]byte(`[
                {"owner": "Kent", "pet": "Shiner"},
                {"owner": "Mary", "pet": "Pumpkin"},
                {"owner": "Mary", "pet": "Tasha"},
                {"owner": "Paige", "pet": "Sushi"},
                {"owner": "Sarah", "pet": "Shiner"},
                {"owner": "Sarah", "pet": "Mr. Wigglesworth"},
                {"owner": "Sarah", "pet": "Pupparoo"},
                {"owner": "Sarah", "pet": "Lucy"}
        ]`))

	var input []Pet
	json.NewDecoder(data).Decode(&input)

	output := make(map[string][]string)
	for _, pair := range input {
		output[pair.Owner] = append(output[pair.Owner], pair.Pet)
	}

	json.NewEncoder(os.Stdout).Encode(output)
}