Bhiṃsana & ˚ka

adjective horrible, dreadful, awe-inspiring, causing fear.

1. bhiṃsanaka (usually combined with lomahaṃsa DN.ii.106 = AN.iv.311; DN.ii.157; Vin.iii.8; Pv-a.22 Thag-a.242 (˚sabhāva = bhīmarūpa); Ja.v.43
2. bhiṃsana Pv.iv.3#5 (+ lomahaṃsa).

the form with -ka is the canonic form, whereas bhiṃsana is younger. See bhiṃsa on connections