List taxonomies in a dropdown in theme’s option panel

A taxonomy is a group of terms. I think you registered a taxonomy Catalogs, and now you want to list all terms in this taxonomy. You do that with the function get_terms(), not get_taxonomies().

So your $wp_tax array should be filled like this:

$wp_tax = array(-1 => 'Choose a category');
$catalog_terms = get_terms('Catalogs');
if ($catalog_terms) {
    foreach ($catalog_terms as $catalog_term) {
        $wp_tax[$catalog_term->term_id] = $catalog_term->name;
    }
}