Is there any specific list of items for this? Or is any food in general?

If you're going for the Gourmand achievements any and all foods are acceptable and count towards the achievement.

That's what I thought, just wanted to make sure. Thank you!

What they mean by unique is that you can only feed one certain food item to count. For example if you feed your pet 5 apples, this only counts as one item for the gourmand achievement. To count you need to feed it an apple, an orange, a pear, a hotdog - which would count for 4 items.

List of all food items. Also beware of filled bottles/flasks unless you really want a creepy camel ;)

Thank you both. This helps quite a bit.