I remember playing a game years ago, early 2000s late 90s and I think it was on the N64. I distinctly remember a mission where you (playing as I guy) had to go from this old saloon or other western themed wooden building, and go up to a cave, pick up some explosives/dynamite/TNT and then use it to destroy some rocks that were blocking the way. While the building looked western, I don't think the game was, but it did have some old looking vehicle for the player to drive (by the saloon on that mission).
The enemies in the game were green spiders, I believe with purple legs, they came in different sizes, when you killed a large spider, it would break down into in to a couple of medium sized spiders, which would then each die in to some small sized spiders. On the mission I talked about, I remember 2 or 3 of the large spiders starting down the road and moving towards you.
If it helps, I remember the way the guy got into the car, he would jump onto the car and fall through the roof, which turned transparent when he was sitting in it and driving it.