Hindus does not know of the sacrifice of animals in the Vedas. There are references in the Rig Veda and others about the sacrifice of the animals.
The horse sacrifice is a well-known ritual from the Rig Veda—though this has pretty much ceased to be practiced altogether. There is also mention of the sacrifice of a goat. In the Yajur Veda, there is an entire section devoted to the rituals of animal sacrifices—though the sacrifice of animals in some cases does seem to be optional.
Animal sacrifice in the Rig Veda
1.162.2-5 The dappled goat goeth before the Courser, covered with trappings and with wealth, the grasped oblation. The dappled goat goes straightforward, bleating, to the place dear to Indra and to Pusan. Dear to all the Gods, this goat, the share of Pusan, is first led forward with the vigorous Courser While Tvastar sends him forward with the Charger, acceptable for sacrifice, to glory.
1.162.6,9-11 (the horse sacrifice) those who prepare the cooking vessel for the Steed…what part of the Steed’s flesh the fly hath eaten, or is left sticking to the post or hatchet, Or to the slayer’s hands and nails adhereth…Food undigested steaming from his belly and any odour of raw flesh remaining This let the immolators set in order and dress the sacrifice with perfect cooking What from thy body shich with fire is roasted, when thou art set upon the spit.
1.162.12 They who observing that the Horse is ready call out and say, the smell is good; remove it; and craving meat, await the distribution, may their approving help promote our labour.
1.162.18 Cut ye with skill so that the parts be flawless, and piece by piece declaring them dissect them.
1.163.12 The strong Steed hath come forward to the slaughter, pondering with a mind directed God-ward. The Goat who is his kin is led before him.
1.172.4 The horse neighs, the steer bellows before being sacrificed
3.2.7, 3.6.6 Horse sacrifice
Here are a few references from the Yajur Veda as well.
Yajur 1.8.21.e To the Asvins he sacrifices a dusky, to Sarasvati a ram, to Indra a bull
Yajur 1.3.1.c He who hates us and whom we hate, here do I cut off his neck…
Sacrifices were done as 1) gift offerings to the gods 2) as a way of gaining strength 3) and for accomplishing things that one wants through spiritual power (curses).
The flesh of the victim was offered partly as a burnt offering and partly was eaten by the priests (who were not vegetarians).
Hinduism is not Vedic religion or Santana Dharma. All this Vedic sacrifice of animals belongs to the Vedic era. Hinduism is not Vedic religion or Santana Dharma. Hinduism is not a religion. Rather it is a group of religions found within India that share common beliefs while still remaining very different.