Mine is the culmination of an ongoing in-joke between me and Ryan.
Several years ago, I called Ryan "a fact-checker crossed with a ferret", which was my way of complimenting how quickly he could confirm/deny the armchair quarterbacking, wild hypotheses, and false beliefs that often occur on this forum.
He took my compliment and listed it under his Interests (under his avatar). The "fact-checking ferret" thing came up on occasion over the years but then didn't get mentioned for quite some time.
About a year ago, I posted some statistical analysis in a thread about plane crashes and provided citations for all of my statistics. He sent me a PM that asked, "Who's the fact-checking ferret now?" and then attached a picture of a ferret wearing aviator gear.
I thought it was perfect (and adorable), so I made it my avatar.