This is the Messenger Anon, a hivemind of 3 different people that travel throughout Tumblr and deliver mail.
They don't recognize themselves as different people and will often self-refer using "we" or "us" (They use "We/Us" for thoughts and speech, and use "I/me" when referring to the actions of their head)
Bit more info down below
FORM 1:
About what you'd expect of an anon. Humanoid, fleshy figure with few distinct features and sunglasses
They head the most, given that most of their recipients are human or humanoid
FORM 2:
They're a shorthair gray cat with sunglasses. Their species is unknown
They head the least due to difficulty carrying packages, but are very useful in navigating smaller spaces
FORM 3:
Mail truck transformer. They did come from Cybertron, but have since assimilated into the Mail collective
They head exclusively in the transformers universe. Even then, they are rarely the head