My 10th level PCs' giant allies have chased a traveling necromancer into an underground ruin. Since the giants don't fit, it's up to the PCs to go in, find the necromancer, and interrogate him as to what his cult is up to. Shortly after entering, they will encounter the necromancer's familiar, a talking ferret who will pretend to just be a cute furry friendly cave dweller who can lead the PCs to the necromancer. Instead, it will lead them to every hazard the dungeon has until the PCs wise up and catch on. 

I need help populating the dungeon with these monsters and hazards. Can you guys help me with suggestions of appropriate monsters/traps and what they're doing there?

Is this a random ruin the necromancer stumbled upon, and he's going into it blind? Or is it a lair or place he set up as a trap?

Assuming that the ruins are random generic ruins, what is the environment like? The things that would live in that ruin would be things that would live in that environment. Is it wet? Molds/slimes/oozes & fungus creatures. Is it a ruin in a forest? Forest creatures that would prefer an underground nest (like insects, standard and giant). Etc.

Also, how big is the ruin?

Quote from Xalthu >>

Is this a random ruin the necromancer stumbled upon, and he's going into it blind? Or is it a lair or place he set up as a trap?

This is an important question. 

If it's just random ruins there would be creatures related to the area like what Sposta said.

If the necromancer lead them this place that he set up as a trap, it could be all kinds of dangerous with preset traps and undead minions.

The other thing that should be considered is how long the Necromancer has had to dig in.

Random ruins, not a preset trap. I had no other pre-decided notions.

Time for the BBEG does allow for some customization of the default layout and addition of few traps seems limited. That leads me to think that the BBEG only has what was already on hand in the old ruin and that's about it. So, the next link in the chain might be: What was the purpose of the ruin before became a ruin?

Your Necro may have access to Guards and Wards, or some other magical assistance in protecting the area. Also, how would the Necro get into the area past all of the existing badness? Was the ruin "luckily" an old elven or dwarven crypt that your Necro would be able to capitalize on? 

Quote from Celerior >>

Random ruins, not a preset trap. I had no other pre-decided notions.

 
This is (most?) of what many of us will need some input from you about before we can give you truly informed ideas. (Help us help you as it were.)

• How many sessions do you want them to spend in those ruins? That will dictate their size, like was it a watchtower or outpost, or a whole city.
• What type of ecosystem are those ruins in? You know how monsters have a list of which environments they inhabit: ARCTIC, COASTAL, DESERT, FOREST, GRASSLAND, HILL, UNDERDARK, UNDERWATER, URBAN? It's your campaign setting, you gotta tell us where the ruins are located. Otherwise we could tell you to populate them with albino [Tooltip Not Found] like in Congo and then you tell us it’s underwater or that a newer city was built on top of them.
• What did the ruins use to be before they got ruined?
• Are they under or above ground? (Like how many of the ancient Egyptian structures got buried by the desert over time?) Or were the original structures subterranean like a mine or something?
• Have the ruins been reclaimed my nature like many of the Mayan/Incan/Aztec cities have?
• Do people generally know of those ruins or not so much? That would possibly indicate if other “Adventurers” (aka grave robbers) have ever explored (aka looted) them in the past. That will partially indicate  how much treasure is likely to be in those ruins, which usually has some correlation to the danger level the party will face. Or, if you don’t yet know how widely known the ruins are to people, how much treasure do you want the ruins to contain? (It’s not unlike a Chicken : Egg situation, it doesn’t matter which way you get there, just that you do.)
• How old are the ruins? (Are we talking Stonehenge old, or were they ruined last year like in a war or catastrophe or something?)
• How were the buildings ruined, was it a war, natural disaster, abandonment, something else? (For example, most of the Aztec and Incan cities weren’t ruined by the Conquistadors, they just stopped being populated because, like, 80% of their inhabitants died off after Columbus gifted them with blankets Smallpox. The Conquistadors mostly just finished off whoever was left and resisted.)
• You said the party is 10th level, but how many PCs are there and do they have sidekicks or anything?
• How long has the necromancer been down there? (That will potentially inform as to how many traps & undead he has added to the dungeon, and how much of it they have had to clear out for themselves.)

About the only things I can tell you is in there are undead monsters raised from the corpses of those the necromancer had to kill themselves.

1-2 sessions,

forest/grassland/hills, but you can re-skin a monster to suit if unsuitable,

underground, but not sure if buried over time or original,

some undead under there which the necromancer could hide among given his powers of evading hostile undead,

abandonment, several hundred years old, 4 PCs, 1 familiar.

Necromancer has had about 1 week, maybe a bit less.

It would be nice to have some traps that give the PCs an opportunity to use or regret not having some uncommonly used skills.

There are a few hazards I immediately think of that would be clear. Falling stones from crumbling walls, floor collapsing beneath the parties feet, rusted nails/sharp metal that might inflict the poisoned condition... One random one that is more of a long-term hazard is a disease from decayed corpses or refuse from animals that have taken up residence in the cave. If the necromancer has had a week there he could probably set up some trap glyphs and wards too.

Monster-wise, there are some options you can explore that enhance worldbuilding too. What caused the ruins to be abandoned all those years ago, and is it still living there? Undead don't age. Underdark creatures might be comfortable there, so duergar or drow could be interesting. If your necromancer is high level and there is Undead in the ruins it could make a very fortuitous opportunity for your bad guy to grow. 

Maybe it's Strahd's less charismatic cousin Strood's hovel? 

So a smaller dungeon then, maybe up to a dozen areas at most.  

If the ruins are an old tomb (like the Egyptian pyramids), then the builders would have placed traps to discourage looters. However, after a few centuries of not being maintained while creatures were tunneling around could have caused destabilization of the structures that could act like unplanned traps. So proficiency in Mason’s Tools would be a beneficial thing, or something to lament not having.

Apes would work for Forested areas. Any insect swarms, Large or smaller giant insects, or miscellaneous creatures, would work wherever. Pretty much any Large or smaller Plants or Oozes of appropriate CRs would be plausible too. Perhaps a chimera, guardian naga, or Young Dragon (perhaps green) if you want something that would pose a significant threat other than the ‘mancer.

But if you are looking for a combo-package of critters and traps, nothing fits both of those criteria better than Kobolds (except for Dwarves).

I would personally only put undead in there that the Necromancer creates or else they would have likely killed everything else unless it was CR 8(ish) or higher. Remember, MPCs don’t follow the same rules as PCs so the Necromancer could raise monstrosities and such (that often surprises players). On the other hand, if it is an old tomb or something then a mummy or wight would work (with a suitable entourage of bodyguards of course). They likely wouldn’t roam around and would only kill whatever may have wandered into the burial chamber.

Thanks guys, you gave me some great advice.