Evolution says that animals evolved because they had to in order to survive.
No, it doesn't.
What I would like explained from Evolutionists is that if an animal did not have what it took to survive in a hostile environment, how did that animal thrive to produce offspring?
It didn't,
by definition.
In one single generation it had to have mutated offspring while unable to survive.
Nope, that's completely wrong.
Why wouldn't the animal just move to a different environment?
They would if they could.
When a population encounters an environmental change that none can survive, and there's no alternative available, they will all die out. That does happen, but it's obviously not the typical situation.
When a population encounters an environmental change that only a proportion can survive, only that proportion will survive, and only their genetic material will pass to subsequent generations. So the descendant population will, on average, be better able to survive that environment, and so-on.
More generally, the individuals that are able to produce more viable offspring will contribute their genetics disproportionately to subsequent generations, so the population average will move in that direction.
For example, if a population of light coloured moths is reasonably well camouflaged on trees with relatively light coloured bark, but industrial dirt & soot gradually darkens the trees so the moths tend to show up more against the bark and are more likely to be eaten by birds, the darker moths in the population are more likely to survive to reproduce than the lighter ones. This selective predation of the lighter moths will result in the average shade of the population becoming darker over time, so the later population is better adapted to the new situation than the earlier one. If the dirt & soot reduces and the tree bark becomes lighter again, the same process will occur in reverse. This is roughly what happened to the
Peppered Moth over the 19th & 20th centuries.