Fish is the best source of Omega 3 fats with its anti-inflammatory properties and small amounts would have occasionally been eaten as part of a primordial diet. Pilchards are an oily fish and along with sardines, herring,  and salmon contain the most Omega 3 (over 1000mg per 100gms of fish). Whole oily fish are recommended over fish oil supplements as the supplements are often prone to oxidisation and unlike whole fish do not contain the natural form of the anti-oxidant Vitamin E to counteract this, (stripped out during processing and a synthetic, less bioavailable form added back in).  Feed your dog or cat 1 or 2 pilchards a week with meals.