Seen? No, I live in the middle of North West England, not exactly a hotspot for cryptids. :v
Believe? Ehh, not particularly. I find them interesting and how they've affected different forms of media, but most of them are either hoaxes or explained by other things, be they misidentifed animal or natural phenomenon.
I wouldn't say aliens count toward this as the existence of other life in the universe (and the possibility more advanced civilisations have visited us) falls more under science, and I'm a firm believer that we aren't alone. Less so as to whether aliens really have come to Earth though, not until one comes up to me and high fives me, anyway...