I think that there are different usages of "come":
Tentacles come (can be gathered from) from jellyfish. This is a general event that can happen at any time so this sentence can always be in the present tense (as long as jellyfish exist).
That (particular) tentacle came from (was removed at some time in the past and brought here) a (particular) jellyfish. Here we're talking about a single event in the past.