[Varyon]

High TL characters aren’t necessarily fully-equipped soldiers, so even wolves and the like can be legitimate threats. If you need animals that can threaten power armor-clad, heavy weapon-wielding soldiers, extremely heavy enemies (or those that can readily dogpile to effectively become heavy) may be able to pin them down, and if they’re persistent enough may be able to remove enough armor to get to the chewy center, or at least cause enough damage to the armor to seriously inconvenience the character (damaging air intakes can mean needing to remove some armor to be able to continue to breathe, making you more vulnerable later, or can be more immediately dangerous if the atmosphere needs filtration to be breathable). Pests that damage sensitive equipment can also be a serious long-term threat. To be a direct combat threat, you probably need some exotic abilities, like armor-piercing claws/teeth or similar.

To survive against the PC’s long enough to be a threat, large numbers can work if the PC’s have limited area-attack ability (and note simply eating up the grenades/ammo they need later can be a threat). Camouflage and hidden tunnels can also help to get close, but become less useful as TL goes up and gear-assisted detection becomes easier. Surviving in direct sustained combat again needs some exotic abilities, like active camouflage or blaster-resistant carapace.