tiger and thistle

Tiger is from Latin tigrem from Greek τίγρις. The AHD tells us that the Greek word is from the same Iranian source as Old Persian tigra- "sharp, pointed" and Avestan tighri "arrow". The Proto-Indo-European root is *steig- "sharp, pointed".

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.