You should wrap your condition with brackets according to PHP syntax:
if (has_tag('Cat', $post_id)) {
$taganimal = "Cat";
}
If you need to check multiple tags you can use something like
foreach (['Cat', 'Dog'] as $animal) if (has_tag($animal, $post_id)) {
$taganimal = $animal;
break;
}
but it would find only the first tag listed in array in case the post has several of them.