Pet Attack is an addon by dehedbar. I am currently maintaining it for him.
It reminds our forgetful toons that we have a pet that is not attacking while in combat. A moveable message (left-click drag) will pop up and inform you of this fact.
It supports classes with a permanent pet. Frost Mages with Glyph of Water Elemental may see the warning pop every now and then although your pet is attacking. Warlocks using Grimoire of Sacrifice will be reminded at combat start that they are without a pet after which the warning should cease. Also, currently may fire when quest requires you to mount a vehicle-looking into that.
Again, move the message by left clicking on it and dragging it to a different spot.