Mastodon was destryoed by special ability of the Octopus, not by the Octopus itself, I think, so it doesn't count...
on Mastodon is written >>attack<<
so here you are, read the cards you play carefully
(happens to me kinda often as well, so I already learned that)

This seems like a bug to me too, either in the wording or the implementation. My guess is that implementation corresponds to "if the damage from a figure's attack...". So either the card should be changed to read like this or octopus should get credit, since any reasonable notion of causality has to attribute the destruction of the mammoth to the attack of the octopus.