How are you defining humor? We can come up with lots of examples of animals being playful for their amusement. That certainly fits Webster’s definition of humor.
Are you perhaps thinking of intelligent humor? Such as the ability to convey something humorous, like a joke?
I once witnessed chipmunks drinking beer. The group seemed to enjoy watching each other trying to walk straight. Regardless, enjoying drunkeness would seem to indicate a capacity for abstact thought. Abstract thinking implies capacity for humor, imho.
Well, I know our dog uses them as a combination of humour and revenge.
My partner occasionally likes to let one rip onto our Kelpie, Bella. Most times she ignores it partly because a) it’s from her beloved Master b) she’s asleep and c) because it probably doesn’t even compare to the ones she drops, which are silent deadly eye-watering zingers.
A close friend, after a couple of drinks and encouragement from said partner, also felt the need to bestow Bella with a gift. Dog ignores this but after about 20 minutes (during which she’s been building one up), she gets up from her sleep, goes over to our friend, reverses up to him and drops one. Then she sneaks off back to her mat and goes straight back to sleep.
You can’t tell me she wasn’t trying to keep a straight face when the silent deadly was detected!
Portuguese Water Dogs are known for having a very clearly observable sense of humor. One small example: both of my dogs will occasionally come up to me, sit, beg for my attention, and when I look at them … let out a little burp.