Paul Farquhar
Legend
The spell does not say the DM picks the creature, and I have never actually seen that happen in play. It is clear to me that the intent of the spell is for the caster to choose the type of beast, especially since it isn't even an issue of whether the beast actually lives in the area since they are really fey spirits.
Some DMs seem way too quick to limit player agency.
Actually, there are two reasons why the DM chooses:
Legacy: in earlier editions, the summoned animals where actual animals, not fey spirits. Thus, they had to be animals that are normally found in that location: tigers in the jungle, polar bears in the arctic, giant rats down a dungeon etc.
Practicality: Only animals that the DM has stats for can be summoned.