Can summoned monsters, specifically natural creatures such as eagles, be commanded to aid another? I've tried to find the answer myself but haven't succeeded.
Yes, with difficulty.
Celestial/Fiendish animals summoned with Summon Monster have their normal animal intelligence. They don't have languages, but they can be commanded like any animals can, namely, with Handle Animal.
Since the summoned creatures don't have any tricks listed in their descriptions, it's pretty safe to assume that they don't know any. However, they are willing to follow your commands. They can probably be treated as domesticated animals with no tricks known for the purposes of Handle Animal. Thus, you can make a DC 25 Handle Animal check to 'push' them to use the Aid trick and make the Aid Another action. Since nothing in their entry prevents them from using the Aid Another action, they are able to.
In the Summoned creature has animal intelligence, then you are limited to the things that can be done with a Handle Animal, but without a the Animal Handling roll since it will likely speak one of the languages that you are familiar with like KRyan mentioned. It can certainly help you in other ways like providing flanking.
\$\begingroup\$ -1 As was stated in the answer by @DuckTapeal, Celestial/Fiendish animals don't speak any more languages than regular ones do. \$\endgroup\$– MrLemonApr 14, 2014 at 8:02
Summoned Creatures can Aid Another
Unless a creature specifically says it can’t use Aid Another, it can; it’s one of the default, generic actions available to everything.
You should be able to give them the command
In 3.5, creatures summoned with summon monster are intelligent and lingual; if you speak their language, you can command them to do anything. Aiding another is a command they can complete.
In fact, using summons to flank and Aid Another is one of the best ways to get the most out of them. Most of the summons out there tend to have relatively low attack bonuses or save DCs, which make their own attacks less valuable than simply helping real characters.
But Pathfinder changed this, removing summoned creatures’ intelligence and language. This makes them useless, and means there is absolutely no reason to ever bother using the spells. So I cannot more strongly advise you to flat-out ignore that change, because otherwise every time someone casts summon monster, they are shooting themselves in the foot. The summon monster spells were weak in 3.5, and now they just serve absolutely no purpose whatsoever.