<?php
error_reporting(-1);

$arr = [
    'African elephant',
    'Spotted hyena',
    'Snow leopard',
];

$new_arr2 = [];
$new_arr3 = [];

$final_array = [];

foreach($arr as $k){
    $new_arr = explode(' ', $k);
    array_push($new_arr2, $new_arr[1]);
    array_push($new_arr3, $new_arr[0]);

}


for($i = 0; $i < count($new_arr2); $i++){
    if (!in_array($new_arr2[$i], $final_array) || !in_array($new_arr3[$i], $final_array)) {
        $final_array[] = $new_arr3[rand(0, count($new_arr2) -1)] . ' ';
        $final_array[] = $new_arr2[rand(0, count($new_arr3) -1)] . '+';
    }
}

echo '<pre>';
var_dump($final_array);
echo '</pre>';