You must provide a collateral or forget about the loan.
Best collateral around are other crypto currencies. But not all of them will be accepted. People won't accept some new/unstable coins.
Value of a collateral should be ~110% of a loan value.
Why anyone would ask for a loan if he have any altcoins?
Well.. perhaps many reasons for example I can say that let say you have some altcoins but you need some BTCs.
You don't want to sell those coins because you see that the value is rising and you think that this is not a good moment to sell.
So you ask for a loan to keep your coins and payback from some other income. .
I must say there are only few cases when asking for a loan is reasonable.
In most of the cases it is not worth and better to wait for some income or sell your altcoins.