And also according to the AHD, the extended variant *teigs- became Old English þistel and English thistle. Where did the -tel come from?
The more conservative OED says about tiger (for "Zend" read "Avestan"):
(Some have conjectured connexion with Zend tīghri arrow, tighra sharp, pointed, in reference to the celerity of its spring; but no application of either word, or any derivative, to the tiger is known in Zend.)
*steig- is also found in raita.