Tigers aren't mentioned in the Rig Veda Samhita, but they are mentioned in [this chapter](http://www.sacred-texts.com/hin/sbe01/sbe01239.htm) of the Kaushitaki Upanishad, which is part of the Shankhayana Aranyaka of the Rig Veda:

> vṛṣṭirbhūtvā varṣati sa iha kīṭo vā pataṅgo vā śakunirvā **śārdūlo** vā siṃho vā matsyo vā paraśvā vā puruṣo vānyo vaiteṣu sthāneṣu pratyājāyate
>
> And according to his deeds and according to his knowledge he is born again here as a worm, or as an insect, or as a fish, or as a bird, or as a lion, or as a boar, or as a serpent, or as a tiger, or as a man, or as something else in different places. 

They're also mentioned in [this chapter](http://www.sacred-texts.com/hin/yv/yv06.htm) of the Taittiriya Samhita of the Yajur Veda:

> Now the man who is consecrated is a foetus, the consecration-shed is the womb (in which he is). If the man who is consecrated were to leave the consecration-shed, it would be as when a foetus falls from the womb. He must not leave, to guard himself. The fire here is a **tiger** to guard the house. Therefore if the man who is consecrated were to leave (the shed), he would be likely to spring up and slay him. He must not leave, to protect himself. 

They're also mentioned in many places of the Vajasaneyi Samhita of the Yajur Veda, for instance in [this chapter](http://www.sacred-texts.com/hin/wyv/wyvbk19.htm):

> My she, Vishûchikâ, who guards these two, the tiger and
the wolf, the lion and the wingèd hawk, may she guard this man
from distress.

And they're mentioned in many places in the Shatapatha Brahmana, for instance in [this chapter](http://www.sacred-texts.com/hin/sbr/sbe44/sbe44044.htm):

> Now, once upon a time, a tiger killed the samrâg-cow of those (who were sacrificing) with (the king of the) Kesin as their Grihapati. He (the king) said to his fellow-sacrificers, 'What atonement is there for this?' They replied, 'There is no atonement for this: Khandika Audbhâri alone knows an atonement for it; but he certainly desires as much as this, and worse than this, (to happen) to thee.'

And most of all, it's mentioned in countless hymns of the Atharvana Veda, because they're an important threat to be warded against.  For instance, [this hymn](http://www.sacred-texts.com/hin/av/av04002.htm):

> A Charm against tigers, wolves, thieves and other noxious creatures
 
> Three have gone hence and passed away, the man, the tiger, and the wolf. Down, verily, the rivers flow, down-goeth the celestial Tree, down let our foemen bend and bow. On distant pathway go the wolf, on pathway most remote the thief! On a far road speed forth the rope with teeth, and the malicious man! We crush and rend to pieces both thine eyes, O Tiger, and thy jaws and all the twenty claws we break. We break and rend the tiger first of creatures that are armed with teeth; The robber then, and then the snake, the sorcerer, and then the wolf. The thief who cometh near to-day departeth bruised and crushed to bits. By nearest way let him be gone. Let Indra slay him with his bolt. Let the beast's teeth be broken off, shivered and shattered be his ribs! Slack be thy bowstring: downward go the wild beast that pursues the hare! Open not what thou hast compressed, close not what thou hast not compressed. Indra's and Soma's child, thou art Atharvan's tiger-crushing charm.