I have personally made a formal request to get historical meteorological data for the date of the sighting to the responsible authority (INMET). I have yet to receive their reply.
When/if you get this data, consider the location of the weather station (by the airport is a common location) and the sampling rate, i.e. how often the data is logged.
An African species in Brazil (1), with alopecia (2), and conjunctivitis (3), running around loose (4).
You now have four unlikely things to explain, instead of accepting one. My goodness, you're persistent.
As for point 1, there are monkeys that reach up to almost 1,5 meter in Brazil (
Muriqui do Norte), even in the State of Minas Gerais. But they are a critically endangered species (currently. don't know about 1996), their range doesn't really reach Varginha, and an adult monkey of
any species showing up in the middle of the city would be quite unlikely. It is also strange that no monkey was found or seen by anyone else before or after the girls.
As far as I was able to ascertain, there are no labs that do experiments with monkeys in the region. But, if there were, I think it's likely that investigators at the time would have looked into that possibility.
The zoo is nearby, but if there were any animals that escaped from the Zoo, I think that explanation would have been given right away.
But, hey, I guess it could be. Just doesn't sound very likely.