I have heard philosophers propose that thought is dependent upon language: that
The question what the relation is between thought and language is, to my mind, one of the most fundamental issues in contemporary philosophy. That is to say, what one's view is about this matter will profoundly shape one's views on many other topics. What one's impression is of the current state of play will, however, depend upon what one has read. There are, as you say, many philosophers would suppose that thought is somehow dependent upon language. One famous example is Donald Davidson, who argues explicitly for this conclusion in "Thought and Talk". On the current scene, John McDowell is perhaps the most visible proponent of the view. Hilary Putnam has held a version of this view in the last several years, and it can be found as well in the writings of Michael Dummett. I could easily continue.
On the other hand, however, there are plenty of philosophers who reject this view and hold, as you suggest, that the ability to think does not depend upon the possession of language skills. Jerry Fodor, for example, would certainly suppose that pre-linguistic children and many non-human animals are capable of thought. Christopher Peacocke holds a simlar view (see his paper "Concepts without Words", for example). In fact, it's probably fair to say that most philosophers of mind would be in this camp.
More interesting to me, however, is the fact that most linguists, I think, and the overwhelming majority of psychologists would also be in this camp. There is a lot of very interesting work being done in psychology nowadays on infant cognition and conceptual development: At the moment, one hot topic is acquisition of numerical concepts, for exmaple. This work simply presumes that infants are cognitive agents, and the success psychologists have had operating on that assumption gives us good reason to believe it. Moreover, much work in psycholinguistics proceeds on the assumption that word-acquisition at the earliest stages is, for the most part, a matter of associating words with concepts one already has.
It's important to be clear here what one means by "have thoughts". You speak, in part of your post, about "whatever goes through the heads" of platypi. That's one sort of thing one can mean. But you will note that I have tended to speak of whether a creature is a "cognitive agent", by which I mean: Should we really think of the creature as having beliefs, desires, intentions, and the like, that explain (or rationalize) its behavior the way our beliefs, desires, and the like explain (or rationalize) ours? The "should really" here is important, because we do of course explain animals' behavior in pyschological terms all the time. The issue is whether we should take such explanations fully seriously or should instead explain infants or animals' behavior in some quite different terms. The question what infants or animals have "going through their heads" and how similar it is to what goes through normal adult humans' heads is of less interest. As far as the status of animals is concerned, then, the place to look would be e.g. at ethnographic work on the great apes. Such work as with which I am familiar makes an impressive case that apes are cognitive agents in the relevant sense.
Note that this is not to say that the possession of language does not make some important difference to cognition. It pretty plainly does, and it's a nice question exactly what kind of difference that is.