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What happen if I polymorph an ally to assist the group in a fight? Will he end up attacking us, or remember the fact than we are ally and help us?
The only relevant part I can find is this:
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.
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It retains its Alignment and personality.
This part essentially means it does remain your ally even polymorphed. However, most beasts have very low intelligence scores making them incapable of higher reasoning, and most also don't understand any language spoken by PCs, so without the aid of spells like Speak With Animals it is going to be fairly tough to coordinate anything more complicated than 'Orc bad, smash!' with them.
In practice this is very often ignored by both players and DMs (in no small part because roleplaying such a situation is simultaneously difficult and boring when done correctly), but if you're concerned with sticking to RAW and RAI this is a concern.
Because of this (combined with the fact that there are many rather weak beasts), while it can be used as a utility or combat spell it's often better to use it aggressively to turn a potentially dangerous enemy into a fairly harmless critter for a while.
Since the text of Polymorph doesn't clearly answer this, let us look at some other examples. As an extreme example, we have the Feeblemind spell (emphasis added):
On a failed save, the creature's Intelligence and Charisma scores become 1. The creature can't cast spells, activate magic items, understand language, or communicate in any intelligible way. The creature can, however, identify its friends, follow them, and even protect them.
Even with the lowest Intelligence score possible, a creature affected by Feeblemind can still identify its friends. For a more mundane example, we can look at the stats for an actual beast, such as a wolf. A wolf has an Intelligence score of 3, and yet it is clearly able to identify its friends, since wolves live and hunt in packs. In fact, the wolf's stat block even includes a mechanical ability that requires it to be able to identify its allies:
Pack Tactics. The wolf has advantage on attack rolls against a creature if at least one of the wolf's allies is within 5 feet of the creature and the ally isn't incapacitated.
So, nothing about the transformation into a beast or the associated reduction in mental ability scores makes it inherently impossible for a polymorphed creature to distinguish friend from foe. Given that the spell also says the creature retains its alignment and personality, this means it would retain its desire not to hurt its friends and to defeat its enemies.
Polymorphed creature retains its alignment and personality. The creature still acts as an ally. However you may have to compensate for the limited mental skills affecting how it acts. Such as lowering intelligence would mean the character wouldn't have the same capacity for critical thinking and lowering Wisdom would lower its intuition.
