IBE is the heart of t IBE t and X is an unknown quantity; I think the IBEX is a mountain goat of some sort, though whether or not it's native to Tibet I do not know.
Thanks BigMac and Devonian. I had IBE + X, however, I could not see the relevance to the rest of the clue as it would indicate that it is known to Tibet. The IBEX (Capra Ibex) is a bovid mammal with a range and habitat of the European Alps and the Middle East. Thanks anyway.