Whether it is in fact a cat is a little open to question. There is, to be sure, a small animal - but other than that, whether it be cat, dog, fox, or any other number of possible quadruped is, from simply the image perspective, a little ambiguous. Some have specifically linked the animal to the lynx, or even, astronomically, to the constellation of the Little Bear (small dipper).
With regards to the 'tearing down' of the pants, it may be worth remembering that part of the more standard clothing of the times were not pant(aloon)s, but rather what we would these days call stockings, rolled up when the weather proved colder. The animal jumping up may therefore also have been 'playing' with, possibly, a loose thread - or something else, and far more personal and of permanent fixture to the man, if the depiction of the Noblet rendition is considered.
With regards to the 'significance' of the act, part of that can be interpreted in various ways: whether the Fou is considered a foreigner in a village, and to which the local dog keeps away; a friendly companion to the Mat in his ongoing journey into strange territories; or an attack from a wild creature; or even a representation of fear, with the hare striking chilling fear as it unexpectedly jumps upon the poor Fou...