Sayyiduna Zayd bin Thabit (may Allah be pleased with him) said: The two testicles are equal. Makhul (may Allah have mercy on him) said: While circumambulating the House of Allah, I mentioned this to Amr bin Shu'ayb (may Allah have mercy on him). I said: How strange of those who differentiate between the two! We castrated a goat on its left testicle, and it still mated from the right testicle.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16321