The amazing find from websearch yields a mini-picture of a herbarium sheet of Eulophidium ambongense
Holotype of Eulophidium ambongense Schltr. Verified by Perrier de la Bâthie, H., 1950
From: West, sandy forest/wood. [Ouest: bois sablonneux], Manongarivo (Ambongo)
The wealth of online material has just help eliminate the O. ambongense possibility. I just have to wait for flowers to confirm if it is O. decaryana or O. aff*. decaryana! [*decaryana has distinctively 5-angular pseudobulbs]
It is definitely a slow grower. Enjoys drying out between watering. Onion-like 1-1.5" succulent pseudobulbs helps it tide-over long drought much better than fellow madagascan euphorbias! In fact, a distinct period of drought triggers formation of new buds for next season's growth.