Aliens are large possibility, maybe to the way there depicted in movies, but still a large posibilty, you've got to have an open mind on evolution though, see I think the xenomorphs in aliens are probably one of the more realistic aliens presented in movies, mostly becuase of my open minded look on evolution, see we mostly evolved from simple organisms, flat worms and such and we evolved the way we are becuase of our enviroment, we evolved as are enviroment evolved as are enviroment changed, we changed to adapt to it, those who couldnt adapt died out, xenomorphs could have evolved in a more harsh enviroment that could still sustian life, the simple organisms would have had to evolve differently, depending on that enviroment, Acidic blood, sharp claws, large tail, the ability to climb, poor vision with other senses heightend and dark skin tell alot about this creatures enviroment, if this creature was to exist in the real world on a planet would probably come from a dark planet, extremly primitive and hostile, there wouldnt be a great need for sight so the planet maybe far from the nearest sun, so were looking at a cold climate. The creatures hit and run tactics and there need to hunt and gather things into the saftey of there cave or hive to there queen shows that there not the highest creature on the food chain on that planet, I could go on about this for hours, but a creature like this isnt unfeasable.
Maybe theres a planet out there that evolved almost the exact same way our planet did but never got hit by no astriod so dinosaurs may never have went extinct, its possible.
There a book I once heard that I think was called star master were a good zooms off to out of space to find the star master(God)and on his journeys he comes across different planets with inteligent life forms that are not far from us but evolved from different organisims, such as star fish like creatures and such, this made reflected on there appearance and the way the moved and talked, this I think would also not be far from the truth of what aliens may be.
But you've got to understand, these aliens that are out there probably wouldnt have the technology we have, maybe there far behind or far foward then us, see these aliens may have never invented the things we have and they may have technology we wouldnt ever have though up, they may never had found a way of extracting and using fossil fuels, never found a use for electricity, never realised that a circle object would move things easier, they will probably not have the languages we have, or maybe they will have some think close to our languages under different names.
A problem is also in the fact that if we ever did finally set forth out of this galaxy, created a ship that could go to other planets nd galaxys or created a telescope that could see other planets in detail, if these aliens had worse technology then us then it wouldnt sound far fetched that we'd probably end up invading there planet for resources and land, then we would probably become the space aliens depicted in the movies, and vice versa if another planet found a way of doing the same (getting to other planets ect) who's to say there gunna come in peace?.
As for big foot, I think its possible but not as we see it, as the big man ape yeti roaming the forest, it would be possible that a few neanderthal survived the ice age and theres a few here and there roaming, but that thoery as alot of holes, as surely they'd had been spotted roaming into a town or city during a hunt, surley they'd had been found by some one else other then attention seeking drunks who seem to carry a camera with them every were they go yet carnt seem to actually use the damb thing.
Locheness monster, this story could be more correct then we think, but again in ways different to the myth, maybe a large deep sea animal surfaced a few times in certain parts of the world, one of the places being lochness, maybe a species that lost its way, this creature hasnt just been cited in the loche apparently theres a good few places with simlier sightings of a simlier creature, probably a species that usually live under in the deep sea as its normal habitat, but there are many holes to that theory too, given the reported shape of the creature it doesnt look like type of creature built for the deep sea.