There is no such animal.
The wild Haggis of Scotland only eats underpants, which is the reason why Scotsmen never wear anything under their kilts. This goes back a very long way in the mists of time when Hawkeye the New of the Clan Macgrumpy was cornered by a pack of wild Haggis. Some tales say he was never the same man again, and spoke for the rest of his life in rather a high-pitched voice.
This animal is called the Greater Panteater(Myrmecophaga pantaloona), or commonly just the "Panteater", and is one of the most voracious clothovores known to man(others include the Mitten Kitten, who takes children's mittens when they're at school, and the Dryer Spider, who uses the left side of socks to spin his cobwebs).
The Panteater has no teeth, but a protruding snout with an extremely long, thin, quick and agile tongue that it uses to take hold of its only source of food: human pants. This usually has an immediate psychological effect on the person who thus finds their pants gone, with certain levels of embarrassment and panic, etc. being among the most common reactions(those who are more accustomed to the Panteater's ways have been shown to be able to withstand these events with more grace and demeanour, however).
With this, the Panteater is not genuinely malevolent, and only takes your pants because it is hungry, generally leaving the rest of your body unharmed; thus, the only real danger inherent in a Panteater "attack" comes from the undue stress and possible embarrasment or public/social humiliation that might come associated with suddenly finding yourself without any pants.
Despite having an extremely impressive range, and basically cohabiting with wherever pants-wearing humans can be found, the Greater Panteater is very elusive and actual sightings are infrequent-to-rare; but if you've ever seen anyone with their pants suddenly around their ankles, or even stripped clean down to their underoos, then you can be sure that a hungry Panteater has struck again(in the case of the pants-around-the-ankles scenario, the attack was unsuccessful and the Panteater must find another prey to stalk, as they rarely hunt the same victim twice after losing the element of surprise, etc).
Because the diet of the Panteater consists exclusively of human pants, scientists have long debated on what these animals might have eaten before-or-in-the-absence-of these garments; the general consensus is this is an example of extreme and near-perfect pseudo-symbiotic co-evolution between Man and the Panteater, with the Panteater being shown to be able to digest and live off other forms of sustenance, but preferring human pants above all else.
In any event, the foragings of the Greater Panteater is one of the reasons why a great many cultures from around the world never really took to wearing leggings or "pants" of any kind, with the Egyptians, Greeks, Romans, Scots, many native tribes, etc. all wearing some form of tunic, robe, kilt, loincloth, fur-wrapping, etc. etc. etc. instead of a full-length set of pant-trousers(which would surely attract the unwanted attentions of the hungry Panteater).
Protection from the Panteater was also one of the main reasons why women in modern western cultures were encouraged not to wear pants, as was the custom of dressing children of both sexes in identical pants-less clothing such as swaddling cloths, toddler outfits, and even into childhood with more "sissy"-type clothing(as can be seen during the Victorian era, etc); again, there was no real inherent *danger* at risk from this animal, but it was a safeguarding measure nonetheless.
The development of "shorts" or "short-pants"(to use the more accurate term) was a deliberate initiative that was meant to curtail the attack of the Panteater, and they are certainly effective to some degree; however, the Panteater has also been known to adapt and will take even these "short-pants" for its sustenance if there is nothing else available. Related to short-pants, but in a different manner, are the variation known as "cut-offs", which are actually the remains of a full pair of pants that did not remove cleanly from a person during a Panteater attack, sometimes as a result of resistance on the part of the prey; in either case, said victim is left with nothing but a pair of threaded leggings where a full-on set of pants used to be and, in fact, some persons have been known to wear these "cut-offs" with pride as a show of their somewhat-victory over the Panteater.
In some cases, overzealous or super-hungry Panteaters might even strip a person of their clothes entirely buck-naked; these animals did not mean to, of course, because what would a Panteater want with a piece of clothing like a jacket that it can't eat? But it has has been known to happen, especially during times when the Panteater might be caught off-guard or otherwise surprised; of course, this ultimately causes some level of embarrassment on the Panteater as well, for a lack of hunting finesse in its intended prey, etc. but it still happens. As such, "streaking" crazes and events are widely suspected to be due to migratory shifts in Panteater habitats which bring them into contact with unsuspecting pants-wearers who subsequently go berzerk at suddenly finding themselves without pants or clothing.
The Panteater is not to be confused with the Black Pantser(Pantera pantaloona), which is a separate species in the Cat family and takes human pants not as food, but almost as a matter of trophy-habit for lining its den, etc; this particular animal can frequently be heard cackling a hyena-like yell in the departure and aftermath of a successful "pantsing", and will occasionally display the spoils of its victory in a very public place, such as the tops of trees or(in this day & age) flagpoles, which seems to be a method of showing their prowess to other members in its species.
Otherwise, an animal that eats only *plants* is called an herbivore(from "herb" for green, non-woody plants and "vore" for swallow/devour, etc).