I play a druid character who regularly uses conjuration spells. To make life easy on my DM I prepared a cheat sheet of all the summonable subjects. It's just a little sheet that has AC, relevant abilities and a hit point marker for each summoned creature. If there are 8 creatures summoned for that CR, then there are 8 hit point markers. He usually has me run them for him, but he still makes the decision. Each entry on the sheet has key words that can help him make a quick judgement call: underground, flying, darkvision, poison, etc.
When I cast the spell, I tell him what my character is hoping will appear, usually using a keyword. If I am hoping for air support, I might get owls, or bats. For poison, I might get snakes or spiders. Part of the fun of D&D is dealing with surprise results (one of the main reasons the game has dice).
So yeah, if your players intend on making a lot of use of these spells, some prep work ahead of time can be helpful. If you can convince them to do the heavy lifting (as I did for my DM) so much the better.
In general, the DM should make something appear that has some relevancy and helpfulness for the current situation. The player is expending a spell resource, after all.
No; conjure animals summons a specified number of beasts of a certain CR, not a swarm of them
As written, the conjure animals spell summons a specified number of beasts, and a swarm is a collection of an indeterminate number of creatures. As the Monster Manual specifies (p. 337-339), each of the swarms listed there is a "Medium swarm of Tiny beasts"; a swarm is not a single "beast". The creatures within a swarm are not treated as distinct entities when treating the collective group as a swarm.
Thus, a swarm of any sort of animal would not qualify for any of the four options listed in the conjure animals spell.
Furthermore, the "The Nature of Swarms" sidebar in Appendix A (MM, p. 337) clarifies what swarms really represent:
The swarms presented here aren’t ordinary or benign assemblies of
little creatures. They form as a result of some sinister or
unwholesome influence. A vampire can summon swarms of bats and rats
from the darkest corners of the night, while the very presence of a
mummy lord can cause scarab beetles to boil up from the sand-filled
depths of its tomb. A hag might have the power to turn swarms of
ravens against her enemies, while a yuan-ti abomination might have
swarms of poisonous snakes slithering in its wake. Even druids can’t
charm these swarms, and their aggressiveness is borderline unnatural.
It's clear from the rules that swarms aren't designed to be treated like mundane animals, and that they don't simply congregate naturally or for benign reasons. Rather, they're a result of some malevolent influence, disturbing the natural order of things in some way.
The designers have been remarkably consistent in their interpretations of the rules regarding swarms: they are not a single beast.
As of October 2020, this question is explicitly addressed by an official ruling in the Sage Advice Compendium:
Can conjure animals summon a swarm?
No. Conjure animals summons individual creatures, and swarms are
groups of creatures.
This official ruling is simple and straightforward, and matches the rules referenced above.
It also lines up with a number of unofficial rulings on Twitter about using Wild Shape to turn into a swarm - by Jeremy Crawford in January 2016 and again in November 2016, and by Chris Perkins in June 2015 - as well as an unofficial ruling on Twitter by Crawford in January 2015 about whether a swarm of quippers counts as one fish for the purpose of the trident of fish command (which can be used to cast dominate beast on "a beast that has an innate swimming speed").
As you can see, the rules themselves, the official ruling in the Sage Advice Compendium, and a number of unofficial rulings on Twitter all indicate that a swarm is not "one beast", but rather a collection of beasts.
Best Answer
You must summon a demon
Demons are a specific type of creature, and, luckily, D&D Beyond lets you search exclusively for demons. It even lets you limit them by CR, so you can gets lists like all demons of CR 5 or lower. It is worth noting that these lists will include certain variants as separate entries; for example: the summoner variant of demons and the reduced threat variants of monsters; whether these in particular can be summoned is not entirely clear and is left to the GM.
In particular, "demon" is a Monster tag which are briefly gone over in the Monster Manual, page 7:
If a monster is a demon, the demon tag will appear in their statblock; for example, the quasit has a statblock that contains the following at the top:
This tells you that it is a fiend, but in particular it is a demon and a shapechanger for any features that rely on those terms such as the summon greater demon spell.