I've seen it mentioned in few answers here that ghost type Pokémon appear only at night. Is this true?
No, it's not. For starters, here's a Gastly around 4pm in the afternoon.
I've also been playing the game until 2 am every night since the game was released and haven't seen a single ghost-type Pokémon in the wild at night. It is doubtful that time of day is a factor for them, which means they're probably location dependent, just like other Pokémon.
As far as time of day and the appearance of Ghost (or Fairy)-type Pokémon goes, I definitely wouldn't fault anyone for thinking that there was a relationship there. Multiple sites have created articles claiming as much, and it certainly makes sense that there would be a connection. Unfortunately, based on both my personal experience and evidence posted by other users on this site, there just doesn't appear to be one. I personally have caught two at night and one during the early afternoon, and seen another one on my proximity tracker during the day as well. Ellesedil's answer also provides evidence that Ghost Pokémon are not exclusively found at night.
Unfortunately I have no information about locations where Ghost Pokémon appear. I personally have caught Gastlys near a Park, near a historic landmark, and along a nondescript road near a school, so I can't even offer a guess in this regard. If anyone else has any information (preferably from reputable sources) please feel free to post it to augment our understanding.
Well. Ghost pokemons doesnt only appear at night. Today I walked a pretty far distance looking for pokemons, and suddenly a ghastly popped up right infront of me in the middle of the day. I think you can get pretty much any pokemon at any point of the day regardless of your position. Maybe not water pokemons but.. hehe
I've seen a rare chance of getting Ghost Type in the afternoon. None in the morning for me. At night, at least 3 or more are nearby me, so I'd assume more appear at night, but not much during the day. The game has only been out for so long, so their's lots to explore!
Oddly enough I have only seen ghost types during the middle of the day so I am not sure what the spawn conditions are.
Adapted to location is highly debatable. I walked through a complete circuit inside a large cemetary, and caught absolutely none, except for a rattata and a pidgey. I walked enough inside to completely hatch a 2km pokemon and half of a 5km pokemon.
No, they don't, I caught ghastly in the middle of the day.
Ghost types have a spawn during the day, its just the spawn is relatively lower than the one at night which has less to the thought that they only spawn at night. The can be found but you have be a little more vigilant for them.
Any pokemon can spawn at any time of the day and anywhere, I don't believe that any pokemon in the game yet only spawns at a certain area.
I do think that ghost pokemon more frequently spawn at night though.
My experience is certain types of Pokémon pop up in certain area more than other area.. but not necessarily relates to their genre.. (other wise where to find electric Pokémon?) and no, ghost Pokémon doesn't pops up only at creepy places either..just random to time/location in general depend on their rarity. it is more effective to have poke hunters share where they caught most of what Pokémon in what area, then we all can benefit.
So ghost Pokemon are adapted to location just like every other Pokemon. they are usally found in cemetery's, abandon houses, dark. Secluded places like caves they are more commonly found in those type of locations