It's a reference to the trumpeting sound an elephant/mastodon makes, meant as the equivalent to the 'tweet' a bird such as the one in the old Twitter logo makes.

There's nothing more to it than that: A quirky name based on the chosen logo, following the Twitter example.

I agree it wasn't the best word choice, and I can understand why it has been de-emphasised in favour of 'posts', but the reasoning behind it was logical.