Is it okay for European mounts to be kept in a garage (non-air conditioned) long-term? Will they deteriorate at all from the temperature/humidity fluctuations?

I have them outside by the pool area in Florida. They are the shade but otherwise subject to all the elements.

They have held up great - all the teak furniture got weathered but the Europeans look great.

Mike

I have had mine in the garage, some for 30 years; no issues. And we have humidity and heat close to Texas. Almost.

They’ll be fine

A little different environment than you, but I have several that have been in an unheated garage in MN for about 10 years, and they are fine.

The hippo skull that has been in a heated/AC building, well the teeth are all shattered...

The moose antlers that have been outside for 7 years now over the cabin door have some gnaw marks but overall look fine.

If this is in MN with -40 to 105 and humidity swings, you will be fine.