Lotus Cobra

Lotus Cobra's ability isn't a mana ability. Opponents may respond to it before you have that mana.

Lotus Cobra's ability uses the stack and players may respond to it. It isn't a mana ability because the event that causes it to trigger isn't a mana ability.

A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.