There's a lot on the news about dogs in the heat and dogs in cars (and yet every year there are tragedies 
)
But close to my heart is the small fur coated pets . I had guinea-pigs over many years (lost the last old girl in April) but not had rabbits.
Especially if you know someone who has got a new pet and this might be their first summer with them.
They can very quickly and easily over heat and dye from heatstroke . Even if they get to a vet , it is often not possible to revive them.
So:
water , cool fresh water . If they are anything like my piggies they won't drink it but offer it
freeze a few 2 litre bottle of water , wrap in newspaper and tuck into the hutch
floor tiles are good , ceramic keeps cooler
keep them in shade , wood absorbs heat . Little furry animals like to hide themselves , they are prey animals , they like to hide in hay , which of course , insulates
make sure they can move about , don't let one animal trap another in a hiding box
some cucumber or rinse some veg in water , but be aware too much can give them loose bowels , which brings me to.....
flystrike . Grim and horrible . Flies lay their eggs on (mainly rabbits but guinea-pigs can suffer too) especially older or tubby animals who cannot clean themselves
put a towel rinsed in cold water and well wrung out pegged over the top of the cage bars to cool them.
if they get heatstroke they basicaly 'pancake' to get the belly to the floor . Might be sweating, rapid , shallow laboured breathing .
They don't regulate rapid changes of temperature well.
Indoors is easier but a lot of animals live out. We kept ours in a wooden playhouse (with re-inforced mesh lined windows ) and a fan on the wall. It was shaded on three sides so thankfully cooler than outside .
And don't forget , though they love some late evening grazing , the foxes like the oppurtunity to browse too .
My DD and I had 8 guinea-pigs over eight years , summer was always the most worrying time for us .
Hopfully if my rambling saves a Fur Coated Life , it is worth it .