Ethiopian acanthus is native to the highlands of Ethiopia, where it grows in a Mediterranean-type climate with free-draining soil. Apparently used for fencing around vegetable gardens in Ethiopia, it has been introduced into ornamental cultivation in Europe and the USA in recent years.
Most of the online information about Acanthus sennii is from gardening and nursery sites (a search for Acanthus sennii), including subjects such as zone information (USDA zones 8b-10), height (varying from 30cm (1 ft.) to 1.5m (5 ft.), and cultivation. A couple people have also written about their experiences growing it, accompanied by many additional photographs: the Outlaw Gardener and Danger Garden.
Digging a little deeper, I noted a reference to an article titled “Synopsis of the species of Acanthaceae in tropical East and Northeast African and tropical Arabia” published in 2007 within the Kew Bulletin via this page: It’s All In The Name. The author of the latter article came to the same conclusion I did, in that this species is likely named in honour of Gustav Senn.
Information in the Kew Bulletin article refines some of the nursery snippets. For example, this species is native to elevations of 1700-3200m (5600 ft. – 10500 ft.), occurring in: “Juniperus and Podocarpus forest, montane grassland and woodland in forest glades, secondary scrub, roadsides”. The author also gives a conservation assessment of Near Threatened as of 2007, due to the one-two punch of habitat transformation into farmland followed by purposeful removal of it as a “weed” to improve grazing.